rdpsnd_server_handle_messages: change return type

With BOOL as return type it is not possible to differentiate between
success and "no data can be read" (when a channel read would block).

rdpsnd_server_handle_messages returns now int with the following
possible values:

-1 if no data could be read
0 error (like connection close) (formerly FALSE)
1 succsess (also if further bytes need to be read) (formerly TRUE)

Not using -1 for error cases was chosen to be compatible with the BOOL
return values used before.