env-util: Add helper to store current log level

This is useful after a fork but before an exec into a binary that uses
systemd's logging utilities. For example, this should be used in dbus
services that fork off worker processes: currently, the log level set by
the LogControl dbus API will be lost because of the exec, and the worker
process will not have the correct log level set.

src/basic/env-util.h

@@ -72,6 +72,7 @@ int set_unset_env(const char *name, const char *value, bool overwrite);
 int putenv_dup(const char *assignment, bool override);
 
 int setenv_systemd_exec_pid(bool update_only);
+int setenv_systemd_log_level(void);
 
 /* Parses and does sanity checks on an environment variable containing
  * PATH-like colon-separated absolute paths */