shared - reinstate plymouth_running()

This will be used by fsckd. This reverts part of
031886edfc.

src/shared/util.c

@@ -4228,6 +4228,10 @@ bool nulstr_contains(const char*nulstr, const char *needle) {
         return false;
 }
 
+bool plymouth_running(void) {
+        return access("/run/plymouth/pid", F_OK) >= 0;
+}
+
 char* strshorten(char *s, size_t l) {
         assert(s);
 

src/shared/util.h

@@ -549,6 +549,8 @@ int kill_and_sigcont(pid_t pid, int sig);
 
 bool nulstr_contains(const char*nulstr, const char *needle);
 
+bool plymouth_running(void);
+
 bool hostname_is_valid(const char *s) _pure_;
 char* hostname_cleanup(char *s, bool lowercase);