init: log when 'user' is unspecified

NOTE: in master, but should be submitted in AOSP.
Waiting to hear from security folks. Also might
need cleanup.

Not currently done. Seems errorprone.

Bug: 276813155
Test: boot, check logs
Change-Id: I7cbc39b282889dd582f06a8eedc38ae637c8edec
diff --git a/init/service_utils.h b/init/service_utils.h
index d4143aa..27ec450 100644
--- a/init/service_utils.h
+++ b/init/service_utils.h
@@ -91,11 +91,13 @@
     IoSchedClass ioprio_class;
     int ioprio_pri;
     std::vector<std::pair<int, rlimit>> rlimits;
-    uid_t uid;
+    std::optional<uid_t> parsed_uid;
     gid_t gid;
     std::vector<gid_t> supp_gids;
     int priority;
     bool stdio_to_kmsg;
+
+    uid_t uid() const { return parsed_uid.value_or(0); }
 };
 
 inline bool RequiresConsole(const ProcessAttributes& attr) {