Add support for customizing the usable nick length.

Add support for customizing the usable nick length.

This adds a new ISUPPORT token, NICKLEN_USABLE which is strictly an informative value.
NICKLEN is always the maximum runtime NICKLEN supported by the IRCd, as other servers may
have their own usable NICKLEN settings.  As NICKLEN_USABLE is strictly informative, and
NICKLEN is always the maximum possible NICKLEN, any clients which depend on NICKLEN for
memory preallocation will be unaffected by runtime changes to NICKLEN_USABLE.

The default NICKLEN is 50; the default serverinfo::nicklen in the config file is set to
30, which is the NICKLEN presently used on StaticBox.