log: introduce log_parse_environment_cli() and log_setup_cli()

Presently, CLI utilities such as systemctl will check whether they have a tty
attached or not to decide whether to parse /proc/cmdline or EFI variable
SystemdOptions looking for systemd.log_* entries.

But this check will be misleading if these tools are being launched by a
daemon, such as a monitoring daemon or automation service that runs in
background.

Make log handling of CLI tools uniform by never checking /proc/cmdline or EFI
variables to determine the logging level.

Furthermore, introduce a new log_setup_cli() shortcut to set up common options
used by most command-line utilities.

src/basic/log.c

@@ -1146,16 +1146,20 @@ static int parse_proc_cmdline_item(const char *key, const char *value, void *dat
 }
 
 void log_parse_environment_realm(LogRealm realm) {
-        /* Do not call from library code. */
-
-        const char *e;
-
         if (getpid_cached() == 1 || get_ctty_devnr(0, NULL) < 0)
                 /* Only try to read the command line in daemons. We assume that anything that has a
                  * controlling tty is user stuff. For PID1 we do a special check in case it hasn't
                  * closed the console yet. */
                 (void) proc_cmdline_parse(parse_proc_cmdline_item, NULL, PROC_CMDLINE_STRIP_RD_PREFIX);
 
+        log_parse_environment_cli_realm(realm);
+}
+
+void log_parse_environment_cli_realm(LogRealm realm) {
+        /* Do not call from library code. */
+
+        const char *e;
+
         e = getenv("SYSTEMD_LOG_TARGET");
         if (e && log_set_target_from_string(e) < 0)
                 log_warning("Failed to parse log target '%s'. Ignoring.", e);
@@ -1430,3 +1434,11 @@ void log_setup_service(void) {
         log_parse_environment();
         (void) log_open();
 }
+
+void log_setup_cli(void) {
+        /* Sets up logging the way it is most appropriate for running a program as a CLI utility. */
+
+        log_show_color(true);
+        log_parse_environment_cli();
+        (void) log_open();
+}

src/basic/log.h

@@ -84,8 +84,11 @@ void log_close(void);
 void log_forget_fds(void);
 
 void log_parse_environment_realm(LogRealm realm);
+void log_parse_environment_cli_realm(LogRealm realm);
 #define log_parse_environment() \
         log_parse_environment_realm(LOG_REALM)
+#define log_parse_environment_cli() \
+        log_parse_environment_cli_realm(LOG_REALM)
 
 int log_dispatch_internal(
                 int level,
@@ -341,3 +344,4 @@ int log_syntax_invalid_utf8_internal(
 #define DEBUG_LOGGING _unlikely_(log_get_max_level() >= LOG_DEBUG)
 
 void log_setup_service(void);
+void log_setup_cli(void);