Block switching to same user as an user occupying current seat

Block switching to same user as an user occupying current seat

Switching to same user as the current user raises a abnormal behavior,
failing on re-login process. This is due to the inconsistance between
the tlm-sessiond daemon lifecycle and the systemd-logind's logout
procedure.
The standard solution is to receive a sessionTerminated signal from the
systemd-logind and sync it to the re-login procedure of TLM, but it
will take many structure changes of tlm-sessiond.
This patch just blocks switching to same user, preventing the abnormal
case with just a small effort.

 * src/daemon/tlm-seat.c|h: tlm_seat_get_occupying_username() is added.
 * src/daemon/tlm-dbus-observer.c: _is_valid_switch_user_dbus_request()
   is added and used in _process_request().

In addition, several comments and debug logs are given.

Change-Id: I9238098771bcce2b40aa89da47e9391ff7556922