string-util: beef up string_is_safe()

This tightens the checks of string_is_safe() and then adds flags to
relax certain aspects of it.

This does alter the rules on certain strings we pass a bit. We mostly
tighten the rules (but I think it's find and good) but we relax them on
others.

I let claude review the changes in behaviour for the various call sites
that I made. It summarized things in this table:

  ╭───────────────────────────────────────────────────┬──────────────────────────────────────────────╮
  │ CALL SITE                                         │ EFFECTIVE DELTA                              │
  ├───────────────────────────────────────────────────┼──────────────────────────────────────────────┤
  │ src/basic/syslog-util  log_namespace_name_valid   │ +UTF-8 required (globs already blocked)      │
  │ src/bootctl  --efi-boot-option-description        │ RELAXED: '\' and quotes now permitted        │
  │ src/core/dbus-manager  pretimeout governor        │ +UTF-8, +no-globs                            │
  │ src/core/load-fragment  ExecStart= path           │ +UTF-8, +no-globs                            │
  │ src/core/main  pretimeout governor (kcmdline)     │ +UTF-8, +no-globs                            │
  │ src/core/service  sd_notify STATUS=               │ +no-globs (ASCII-only preserved)             │
  │ src/home/homectl  --<identity field>=             │ empty now REJECTED; +UTF-8                   │
  │ src/libsystemd-network  dhcp_option_parse_string  │ (equivalent, just explicit)                  │
  │ src/libsystemd-network  sd_dhcp_server boot_fname │ ""→NULL coerced; else equivalent             │
  │ src/libsystemd/journal  SYSLOG_IDENTIFIER fb      │ +UTF-8, +no-globs                            │
  │ src/libsystemd/sd-json  SD_JSON_STRICT strings    │ +UTF-8 required                              │
  │ src/login/logind  session desktop=                │ +UTF-8 required                              │
  │ src/pcrlock  EFI variable string                  │ +UTF-8                                       │
  │ src/pcrlock  EFI action string                    │ RELAXED: empty + '\' now ok; +UTF-8          │
  │ src/resolve  dns-delegate id (from filename)      │ +UTF-8, +no-globs                            │
  │ src/shared/boot-entry  boot_entry_token_valid     │ (equivalent)                                 │
  │ src/shared/conf-parser  section header            │ +UTF-8, +no-globs                            │
  │ src/shared/conf-parser  CONFIG_PARSE_STRING_SAFE  │ +UTF-8 required                              │
  │ src/shared/kbd-util  keymap_is_valid              │ (equivalent; folded into STRING_FILENAME)    │
  │ src/shared/tpm2  nvpcr name                       │ +UTF-8 required                              │
  │ src/shared/vconsole  x11 layout/model/variant/opt │ +UTF-8, +no-globs                            │
  │ src/systemctl  --kernel-cmdline=                  │ +0x7f DEL rejected; empty path split out     │
  │ src/veritysetup  salt=                            │ RELAXED: safety check removed entirely       │
  │ src/vmspawn  --ssh-key-type=                      │ +UTF-8 required                              │
  ╰───────────────────────────────────────────────────┴──────────────────────────────────────────────╯

src/basic/string-util.c

@@ -1099,25 +1099,40 @@ int strdup_to_full(char **ret, const char *src) {
         }
 };
 
-bool string_is_safe(const char *p) {
-        if (!p)
+bool string_is_safe(const char *p, StringSafeFlags flags) {
+
+        /* Baseline checks are:
+         *   • No control characters (i.e. 0…31 + 127)
+         *   • UTF-8 valid (well, technically we skip this test if STRING_ASCII is set, since that is a tighter test)
+         */
+
+        if (FLAGS_SET(flags, STRING_ALLOW_EMPTY) ? !p : isempty(p))
                 return false;
 
-        /* Checks if the specified string contains no quotes or control characters */
+        if (!FLAGS_SET(flags, STRING_ASCII) && !utf8_is_valid(p))
+                return false;
 
         for (const char *t = p; *t; t++) {
-                if (*t > 0 && *t < ' ') /* no control characters */
+                if ((*t > 0 && *t < ' ') || *t == 0x7f) /* never allow control characters */
                         return false;
 
-                if (strchr(QUOTES "\\\x7f", *t))
+                if (!FLAGS_SET(flags, STRING_ALLOW_BACKSLASHES) && *t == '\\')
+                        return false;
+
+                if (!FLAGS_SET(flags, STRING_ALLOW_QUOTES) && strchr(QUOTES, *t))
+                        return false;
+
+                if (!FLAGS_SET(flags, STRING_ALLOW_GLOBS) && strchr(GLOB_CHARS, *t))
+                        return false;
+
+                if (FLAGS_SET(flags, STRING_ASCII) && (uint8_t) *t >= 0x80)
                         return false;
         }
 
-        return true;
-}
+        if (FLAGS_SET(flags, STRING_FILENAME) && !filename_is_valid(p))
+                return false;
 
-bool string_is_safe_ascii(const char *p) {
-        return ascii_is_valid(p) && string_is_safe(p);
+        return true;
 }
 
 char* str_realloc(char *p) {