net: sockets: tls: Fix TLS POLLHUP notification

The `poll()` function did not report POLLHUP if the peer ended the DTLS
session, making it impossible to detect such event on the application
side.

On the other hand, TLS erroneusely reported POLLHUP along with each
POLLIN event, as the 0 returned by the `recv()` socket call was
wrongly interpreted (it was expected to get 0 in return as 0 bytes were
requested).

Fix this by introducing a helper function to process the mbedtls context
and verify if new application data is pendingi or session has ended.
Use this new function in the poll handler, instead of a socket `recv()`
call, to remove any ambiguity in the usage, for both TLS and DTLS.

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