rtspconnection: Handle closed POST socket in tunneling

rtspconnection: Handle closed POST socket in tunneling

Catch more socket errors.
Rework how sockets are managed in the GSource, wake up the maincontext instead
of adding/removing the sockets from the source.
Add callback for when the tunnel connection is lost. Some clients (Quicktime
Player) close the POST connection in tunneled mode and reopen the socket when
needed.

See #612915