Allow clients to specify an ID via PASS or USER

- PASS [user[@identifier][/network]:]password
- USER user[@identifier][/network] ...

NOTE: There's a slight ambiguosity with the '@' character, which happens
to be a valid character in usernames, but also acts as a marker for the
identifier. Therefore, '@' is considered as part of the username if it's
followed by non-word characters (as in an email address), otherwise as
a marker for an identifier.

This is only an enabler for #343. The rest can be done with modules:
- managing client ID specific playback buffers
- filtering channels based on the client ID

The reason this should be part of ZNC core is that only global modules
have access to OnUnknownUserRaw(), which is needed to capture USER/PASS.
First of all, the aforementioned modules shouldn't be global. Furthermore,
it would be possible to have only one module that parsed and removed the
client ID so that ZNC core woulnd't choke.
Add CIRCNetwork::FindClient(const CString& sIdentifier)