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/bootctl/bootctl.c

@@ -39,7 +39,6 @@
 #include "string-table.h"
 #include "string-util.h"
 #include "strv.h"
-#include "utf8.h"
 #include "varlink-io.systemd.BootControl.h"
 #include "varlink-util.h"
 #include "verbs.h"
@@ -573,7 +572,7 @@ static int parse_argv(int argc, char *argv[], char ***ret_args) {
 
                 OPTION_LONG("efi-boot-option-description", "DESCRIPTION",
                             "Description of the entry in the boot option list"):
-                        if (isempty(arg) || !(string_is_safe(arg) && utf8_is_valid(arg))) {
+                        if (!string_is_safe(arg, STRING_ALLOW_BACKSLASHES|STRING_ALLOW_QUOTES|STRING_ALLOW_GLOBS)) {
                                 _cleanup_free_ char *escaped = cescape(arg);
                                 return log_error_errno(SYNTHETIC_ERRNO(EINVAL),
                                                        "Invalid --efi-boot-option-description=: %s", strna(escaped));