Add support for multi-prefix

Add support for multi-prefix

Add support for multi-prefix to get a full list of modes in NAMES and
WHO replies, updating ircuser objects as appropriate.  This enables
seeing if an operator (+o) also has voice (+v), etc.  In the future,
Quassel could show this information in a smart, useful manner.

This commit limits synced modes to one for 'NAMES' replies to avoid a
warning in older clients.  The next commit will address this.

See http://ircv3.net/specs/extensions/multi-prefix-3.1.html