net: tcp: Add semaphore indicating if transfer is possible

The semaphore is reset when TCP layer would normally reject transfer
request (either due to TX window being full or entering retransmission
mode). Once data is acnowledged, or the reatransmission is done, the
semaphore is set again.

Upper layers can monitor the semaphore with `k_poll()` instead of
waiting blindly before attempting to transmit again.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>