fix miscounting in LUSERS

assorted fixes

The server should never crash during rehash, even if the config is invalid.

Previously, we generated and prepended a long salt before generating
password hashes. This resulted in the hash verification cutting off long
before it should do. This form of salting is also not necessary with
bcrypt as it's provided by the password hashing and verification
functions themselves, so totally rip it out.

This commit also adds the functionality for the server to automagically
upgrade users to use the new hashing system, which means better
security and more assurance that people can't bruteforce passwords.

No need to apply a database upgrade to do this, whoo! \o/

refactor rehash to rely more on server.config

simplify CAS logic a bit

some tweaks to account/channel unregistration

implement #263

add CHANSERV UNREGISTER

frontend for persistent account modes in channels

implement SAJOIN, allow channel founders to join unconditionally

fix #268

initial vhosts implementation, #183

reduce copying of output lines

fix #262