coredump: dump priviliges when processing system coredumps

Let's add an extra-safety net and change UID/GID to the "systemd-coredump" user when processing coredumps from system
user. For coredumps of normal users we keep the current logic of processing the coredumps from the user id the coredump
was created under.

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=87354

src/coredump/coredump.c

@@ -569,9 +569,19 @@ static int change_uid_gid(const char *context[]) {
         if (r < 0)
                 return r;
 
-        r = parse_gid(context[CONTEXT_GID], &gid);
-        if (r < 0)
-                return r;
+        if (uid <= SYSTEM_UID_MAX) {
+                const char *user = "systemd-coredump";
+
+                r = get_user_creds(&user, &uid, &gid, NULL, NULL);
+                if (r < 0) {
+                        log_warning_errno(r, "Cannot resolve %s user. Proceeding to dump core as root: %m", user);
+                        uid = gid = 0;
+                }
+        } else {
+                r = parse_gid(context[CONTEXT_GID], &gid);
+                if (r < 0)
+                        return r;
+        }
 
         return drop_privileges(uid, gid, 0);
 }