No longer treat "registration" as a connection context
Don't use the built-in command handlers
Introduce first iteration of new command handler API
Finish the thought in include/msg.h
msg: Add functions to fill source bits
msg: Add find_command and report_failure
msg: Add some tests to report_failure
Move message propagation to its own function
msg: Allow command handlers to request messages be reprocessed.
Use ulong and uchar instead of unsigned etc.
Make connection contexts just NONE, USER, and SERVER

Any other information about a connection should be handled through
alternate means.
user: Change how user structures are created and destroyed
What is a user state anymore? We don't need this.

Prefer flags for postponing registration.
Add core/pass, implementing (kind of) PASS
msg: Return int from u_repeat_as_*
msg: Check cmd->nargs
Add core/nick, providing NICK
user: Add u_user_try_register
Add core/user, implementing USER
u_user_try_register: Stop if any USER_MASK_WAIT bits set