First step towards message tags extension support
(see http://ircv3.atheme.org/specification/message-tags-3.2).

In particular this commit implements a (very) dummy implementation
sketch of the server-time extension
(see http://ircv3.atheme.org/specification/message-tags-3.2 and #499).
Implemented handle_message_tag_time() for time-server messages.
server-time supported in most named messages.
Organized cap inbound code in inbound.c instead of proto-irc.c.
server-time supported in all named servermsg: ping, error, notice and
authenticate.

The only thing left to do for server-time are the numeric messages.
server-time for all numerical messages.
Now inbound_cap_ls() can enable extensions when a bouncer uses a namespace for
the extension server-time.
A few messages missing regarding server-time.
Fixed a few null pointer derefs caused by dummy code that somehow I
forgot about.
Added preferences options to enable the server-time extension when available.
(Internationalization messages missing.)
Fixed compilation error in VS.
Merge pull request #656 from orium/server-time

Closes #499