Merge branch 'capabilities'

Merge branch 'capabilities'

* capabilities:
  "multi-prefix" capability 2/2: adjust NAME and WHO handlers
  "multi-prefix" capability 1/2: implement complete CAP infrastructure
  IRC_Send_NAMES(): Code cleanup
  New function Client_CapSet() in addition to Client_Cap{Add|Del}
  "CAP REQ" starts capability negotiation and delays user registration
  Correctly handle "CAP END", new client type CLIENT_WAITCAPEND
  Implement core IRC capability handling and "CAP" command
  New "login" source file
  Introduce_Client() => Client_Introduce(), and move it to client.c