Show a nicer message when a protocol is disabled in account add
This adds the disabled protocols' prpl structs to a different linked
list, only used for this lookup. They were previously marked as leaking
by valgrind, so, whatever. I can't free them, since some protocols
memdup() it after attempting to register.
I think disabling the protocols from bitlbee.conf is just stupid and
provides no real benefits, but someone will complain if i get rid of it.
So this just improves the error message to make it less confusing when
someone accidentally uncomments that crap.
