phandler: Add handler for sending sasl mechanism list to the network
saslserv/main: Extract mech list building code into a function
saslserv/main: Send mechlist to the network when it changes
saslserv/main: Send the current mechlist to the network when a server finishes burst
protocol/inspircd: Implement sasl_mechlist_sts()

Message format:
:<source> METADATA * saslmechlist :<mechlist>
Merge pull request #368 from attilamolnar/master+saslmechlist

Add support for pushing the sasl mechanism list to the network (IRCv3.2)