util: add parse_uid

Hand-edited by michich from the following upstream patch:

From 034a2a52ac0ec83e0229941d635d310b23eb04df Mon Sep 17 00:00:00 2001
From: Lennart Poettering <lennart@poettering.net>
Date: Fri, 22 Jul 2011 21:01:15 +0200
Subject: [PATCH] sd-login: beef up login api, to add monitoring and
 enumerating

---
diff --git a/src/util.c b/src/util.c
index 328a1ea..2d4f229 100644
--- a/src/util.c
+++ b/src/util.c
@@ -317,6 +317,26 @@ int parse_pid(const char *s, pid_t* ret_pid) {
         return 0;
 }
 
+int parse_uid(const char *s, uid_t* ret_uid) {
+        unsigned long ul = 0;
+        uid_t uid;
+        int r;
+
+        assert(s);
+        assert(ret_uid);
+
+        if ((r = safe_atolu(s, &ul)) < 0)
+                return r;
+
+        uid = (uid_t) ul;
+
+        if ((unsigned long) uid != ul)
+                return -ERANGE;
+
+        *ret_uid = uid;
+        return 0;
+}
+
 int safe_atou(const char *s, unsigned *ret_u) {
         char *x = NULL;
         unsigned long l;
diff --git a/src/util.h b/src/util.h
index f39c01f..9537d13 100644
--- a/src/util.h
+++ b/src/util.h
@@ -136,6 +136,7 @@ void close_many(const int fds[], unsigned n_fd);
 int parse_boolean(const char *v);
 int parse_usec(const char *t, usec_t *usec);
 int parse_pid(const char *s, pid_t* ret_pid);
+int parse_uid(const char *s, uid_t* ret_uid);
 
 int safe_atou(const char *s, unsigned *ret_u);
 int safe_atoi(const char *s, int *ret_i);