basic/log: split out invoked_by_systemd() utility function

src/basic/process-util.h

@@ -190,6 +190,8 @@ int setpriority_closest(int priority);
 
 bool invoked_as(char *argv[], const char *token);
 
+bool invoked_by_systemd(void);
+
 _noreturn_ void freeze(void);
 
 bool argv_looks_like_help(int argc, char **argv);