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>
protocol: Add is_extban() to allow modules to check if the ircd considers a mask an extban
chanserv/quiet: Don't try to expand extbans
chanserv/ban: Don't try to expand extbans