net: app: TLS client handling issue when server closes connection

The client TLS code did not handle server issued close properly.
Now the connection is terminated properly and TLS thread is left up to
wait more requests from the user.

This commits adds new boolean field to net_app context. Because there
are already multiple boolean flags there, convert them all to bitfields
to save space.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>