user-util: add get{pw,gr}{uid,gid,name}_malloc() helpers

These are wrappers around getpwuid_r() and friends, and will allocate the
right-sized buffer for this call.

We so far had multiple implementations of a buffer allocation loop
around getpwuid_r() and friends, and they all suck in some way. Let's
clean this up and add a common implementation, and use it everywhere.

Also, be more careful with error numbers, in particular systematically
turn ENOENT into ENOSRCH (the former is what is returned if /etc/passwd
is absent, which we want to consider identical to user not existing,
which is ENOSRCH). We so far did this at some invocations, but not all.

There are some invocations of getpwuid() left in the codebase. We really
should fix those too, and have a single unified implementation of the
logic, but those are not as trivial to convert, so left for another
time.

src/basic/user-util.h

@@ -158,3 +158,9 @@ static inline bool hashed_password_is_locked_or_invalid(const char *password) {
  * Also see https://github.com/systemd/systemd/pull/24680#pullrequestreview-1439464325.
  */
 #define PASSWORD_UNPROVISIONED "!unprovisioned"
+
+int getpwuid_malloc(uid_t uid, struct passwd **ret);
+int getpwnam_malloc(const char *name, struct passwd **ret);
+
+int getgrnam_malloc(const char *name, struct group **ret);
+int getgrgid_malloc(gid_t gid, struct group **ret);