[SSSD] Append PID to sbus server socket name, let clients use a symlink

[Jakub Hrozek]

[PATCH 1/2] Add option to follow symlinks to check_file()
This functionality will be used later on

[PATCH 2/2] Append PID to sbus server socket name, let clients use a symlink
Only the Data Provider uses a symlink now -- I don't think it makes
sense for monitor, it cannot be restarted anyway. The proxy provider
spawns the proxy_child processes, so if it goes away, so do the
children.

Related question -- the clients now only reconnect when the dbus connection
goes away. Would it make sense to explicitly instruct them to reconnect when
monitor kills the server or perhaps when a new server is registered? This
might help the clients reconnect more seamlessly when the server does not
go away after receiving the shutdown signal.