tree-wide: unify logging of "Must be root" message

Let's unify this in one call, generalizing must_be_root() from
bootctl.c.

src/basic/process-util.h

@@ -138,3 +138,5 @@ static inline bool pid_is_valid(pid_t p) {
 int ioprio_parse_priority(const char *s, int *ret);
 
 pid_t getpid_cached(void);
+
+int must_be_root(void);