util-lib: move runlevel_to_target() to shared/unit-file

It if of course related to /proc/cmdline parsing, but is higher-level
functionality built on top of it. It should be in shared/ because it
is something to be used by pid1 and related utilities, not something for
level-level libraries.

src/basic/proc-cmdline.h

@@ -28,7 +28,6 @@ char *proc_cmdline_key_startswith(const char *s, const char *prefix);
 bool proc_cmdline_key_streq(const char *x, const char *y);
 
 int shall_restore_state(void);
-const char* runlevel_to_target(const char *rl);
 
 /* A little helper call, to be used in proc_cmdline_parse_t callbacks */
 static inline bool proc_cmdline_value_missing(const char *key, const char *value) {