nss: only read logging config from environment variables

log_parse_environment() uses should_parse_proc_cmdline() to determine whether
it should parse settings from the kernel command line. But the checks that
should_parse_proc_cmdline() apply to the whole process, and we could get a positive
answer also when log_parse_environment() was called from one of the nss modules.
In case of nss-modules, we don't want to look at the kernel command line.

log_parse_environment_variables() that only looks at the environment variables
is split out and used in the nss modules.

Fixes #22020.

src/basic/log.c

@@ -1189,14 +1189,9 @@ static bool should_parse_proc_cmdline(void) {
         return getpid_cached() == p;
 }
 
-void log_parse_environment(void) {
+void log_parse_environment_variables(void) {
         const char *e;
 
-        /* Do not call from library code. */
-
-        if (should_parse_proc_cmdline())
-                (void) proc_cmdline_parse(parse_proc_cmdline_item, NULL, PROC_CMDLINE_STRIP_RD_PREFIX);
-
         e = getenv("SYSTEMD_LOG_TARGET");
         if (e && log_set_target_from_string(e) < 0)
                 log_warning("Failed to parse log target '%s'. Ignoring.", e);
@@ -1222,6 +1217,15 @@ void log_parse_environment(void) {
                 log_warning("Failed to parse log tid '%s'. Ignoring.", e);
 }
 
+void log_parse_environment(void) {
+        /* Do not call from library code. */
+
+        if (should_parse_proc_cmdline())
+                (void) proc_cmdline_parse(parse_proc_cmdline_item, NULL, PROC_CMDLINE_STRIP_RD_PREFIX);
+
+        log_parse_environment_variables();
+}
+
 LogTarget log_get_target(void) {
         return log_target;
 }

src/basic/log.h

@@ -82,6 +82,7 @@ int log_open(void);
 void log_close(void);
 void log_forget_fds(void);
 
+void log_parse_environment_variables(void);
 void log_parse_environment(void);
 
 int log_dispatch_internal(