SystemServer: Ensure service drop privileges could fail only when root

If we try to launch a lazily-spawned service and the SystemServer as a
(running --user) session leader is running with root permissions, then
if it is instructed to drop the root permissions for a the new service
then it will make sense to abort the entire spawn procedure if dropping
of privileges failed.

For other users, trying to change UID/GID to something else doesn't make
sense (and will always actually fail) as we are already running in non
root permissions, hence we don't attempt to do this anymore.
It should be noted that if an explicit User configuration was actually
specified for a Service to be used with, we would still try to login
with the requested User option value, which would fail when running as
non-root user.

This is useful for example when trying to run the pro utility with pls
to elevate to root permissions, but the session leader is still the same
so trying to "drop" privileges to UID 0 doesn't make sense.

Userland/Services/SystemServer/Service.h

@@ -33,6 +33,8 @@ private:
 
     ErrorOr<void> determine_account(int fd);
 
+    ErrorOr<void> change_privileges();
+
     /// SocketDescriptor describes the details of a single socket that was
     /// requested by a service.
     struct SocketDescriptor {
@@ -74,6 +76,7 @@ private:
 
     // The resolved user account to run this service as.
     Optional<Core::Account> m_account;
+    bool m_must_login { false };
 
     // For single-instance services, PID of the running instance of this service.
     pid_t m_pid { -1 };