server: permit the add_service function to return the created service

server: permit the add_service function to return the created service

returning the created service seems useful:

as the only method to get the freshly created service is by getting the
last item in the services linked list, and this seems to be like an
intrusion to service internal mechanism.

possibly, we could get the service from a connection but this is possible
only from [new_connection|input|connection_closed]_handler_t, but this is
not always practical:
  example: armv7m: add a TCP channel to stream captured trace
           http://openocd.zylin.com/#/c/5345/
           here we poll for trace and broadcast to all connections
           outside of these xxx_handler_t functions

also, storing one of the connections in new_connection_handler_t and get
the service from it is possible, but this will make the code less readable.

Change-Id: I5fef1baecec1e054953c6faf5b99d864ecc97f02
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5717
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins