vapi: add vapi_stop_rx_thread()

vapi: add vapi_stop_rx_thread()

Type: improvement

Allow vapi to signal to an application's RX thread that it should wake
up and exit.

Before disconnecting from VPP's API, libvlibmemoryclient inserts an
rx_thread_exit message into the client's own input queue to cause its
RX thread to wake up from its blocking dequeue and exit cleanly. Add a
function to vapi's API which will allow libvapi client applications
which have an RX thread waiting for incoming messages using vapi_wait()
to do the same thing.

The existing libvlibmemoryclient code which does this was moved to a
separate function and made available for vapi_stop_rx_thread() to call.

Also fixed some inconsistencies in indentation of function prototypes in
vapi.h to make checkstyle.sh happy.

Signed-off-by: Matthew Smith <mgsmith@netgate.com>
Change-Id: I7bbb73470807123cc63ef313cfb91d1fd31b34e5