async_req: check for errors when monitoring socket for readability

async_req: check for errors when monitoring socket for readability

Add an option to wait_for_read_send(), so that the request, upon
calling back, report whether the socket actually contains data
or is in EOF/error state. EOF is signalled via the EPIPE error.

This is useful for clients which do not expect data to arrive but
wait for readability to detect a closed socket (i.e. they do not
intend to actually read the socket when it's readable). Actual data
arrival would indicate a bug in this case, so the check can
be used to print an error message.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=11397

Signed-off-by: Uri Simchoni <urisimchoni@gmail.com>
Reviewed-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Volker Lendecke <vl@samba.org>