tevent: add support for TEVENT_FD_ERROR

tevent: add support for TEVENT_FD_ERROR

After 12 years we finally got TEVENT_FD_ERROR support :-)

TEVENT_FD_WRITE event handlers never get errors reported
instead the event handler is silently disabled.
There are likely callers relying on that behavior, so
we are not able to chance it.

Now TEVENT_FD_WRITE can be used together with TEVENT_FD_ERROR
in order to get errors reported without waiting for TEVENT_FD_READ.

TEVENT_FD_ERROR can also be used alone in order to detect errors
on sockets in order to cleanup resources.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>