On the link established between two clients, a new command have been added.
During the initial handshake, after the $Direction and before the $Key, each
client can send its capabilities to the remote one using a new commmand named
$Capabilities.

Usage:
$Capabilities CAP1[$CAP2....]|

Each string CAP* is a capability supported by the client sending the
$Capabilities string.

List of defined capabilities:

* SEND_WITH_SIZE

  When a download is initiated, usually, the client sends a "$Send|" command.
  If the remote client has the SEND_WITH_SIZE capability, it also supports the
  command "$Send xxx|" where xxx is the number of bytes the local client wants.
  This capabilities has been added for future usage in segmented download.

