Quiesce critical for setting process uid property to -1

Revert the critical in polkit_unix_process_set_property() from commit 2cb40c4d when setting uid to -1. This is the default value, thus g_object_new() will call it with a -1 value when uid is not specified - this happens with polkit_unix_process_new() and polkit_unix_process_new_full().

There is also nothing fundamentally wrong with setting the property back to -1 after it got set to something else -- -1 needs to be handled when doing actual security checks, but not at property set time.

Fixes #77 (closed)