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.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);