Vanilla religious conversion weirdness

[Silversweeper]

Consistent with vanilla, but quite odd as implemented in vanilla:

• ai_flag_refuse_conversion is not checked in stuff like "Convert to attacker's religion" or MNM.3410 ("SRS leaves a holy book"), and quite possibly elsewhere where there's religious conversion by event/decision.

• MNM.3948 (random AI secret conversion) does not check for the character being Zealous or them controlling a religion, both of which seem like things that should make a character rather unwilling to convert (and which would block e.g. converting to an attacker's religion), whether due to their strongly held belief or the temporal power and prestige associated with being a rel head.

• Zealous characters are blocked from converting to an attacker's religion even if their secret religion matches the attacker's public religion. An odd hill to die on.