protocol/inspircd: Fix warnings about ignored login name settings with SASL.

Inspircd's METADATA accountname serves both for bursting existing
account names (netburst and SASL) and setting new ones (from services
only). If atheme is supposed to control account names, it must allow
bursts but not new account names.

Atheme did not distinguish the two types correctly, in particular
detecting SASL logins as new account names.

Tested by Techman.