Limit the number of message targes, and suppress duplicates

Limit the number of message targes, and suppress duplicates

This prevents an user from flooding the server using commands like this:

  PRIVMSG nick1,nick1,nick1,...

Duplicate targets are suppressed silently (channels and clients).

In addition, the maximum number of targets per PRIVMSG/NOTICE/... command
are limited to MAX_HNDL_TARGETS (25). If there are more, the daemon sends
the new 407 (ERR_TOOMANYTARGETS_MSG) numeric, containing the first target
that hasn't been handled any more.

Closes #187.