Fixes for Muslim Invasion CB

=== muslim_invasion CB ===
A few fixes. It also conforms with the vanilla diff wherever possible now.  Muslim vs. muslim invasion is enabled (of hostile denominations only) as it technically was already, but in such a case, it costs 60% more piety on_add than vanilla (800), cancels less decadence, and has very strict requirements upon usage.  Muslim vs. other religious group (aside from pagan) invasions operate exactly as per vanilla with the usual PB restrictions.