First working version of DCC Receive

This finally allows accepting a file via DCC and storing it on the client side.
Note that this is still a very basic implementation, in particular missing

  * Proper status messages (still claiming to receive unknown CTCP, for example)
  * Proper error handling
  * Proper testing
  * Any kind of configuration
  * Any kind of UI besides the simple dialog on incoming files
  * No queuing or any form of transfer management
  * Things like DCC REVERSE

However, it serves as a nice proof-of-concept for the general approach of
passing data directly to a particular client using the usual sync mechanism
(i.e. no special protocol support or additional connections needed). First
client to accept the transfer will get the file.

Note that the *core* is the one handling the DCC connection itself, so things
like port forwarding or firewall support need to be done core-side.