9111 libstand: Fix IP recv timeout

Review Request #860 - Created Feb. 14, 2018 and submitted

Toomas Soome

readip() doesn't, at the moment, properly indicate to callers that it has
timed out. One can tell that it's timed out if errno == EAGAIN when it
returns, but this is not ideal. Restructure it a little bit to explicitly
set errno to ETIMEDOUT if we've exhausted tleft.

I found two places that care about where it timed out or not: sendrecv in
net.c and sendrecv_tftp. Both are structured to pass smaller timeout values
to readip while tracking a larger timeout. Neither of them were able to do
this properly with readip not indicating ETIMEDOUT, so fix it.

While here, straighten out the time (t/t1) usage in sendrecv_tftp.

This would have manifested itself in periodic failures to NFS/TFTP boot for
no apparent reason because MINTMO/MAXTMO were not actually being respected
properly. Problems were not reported with NFS, only TFTP.

Reported by:	sbruno
Reviewed by:	sbruno, tsoome
MFC after:	3 days
Differential Revision:	https://reviews.freebsd.org/D14350

Yuri Pankov
Andy Fiddaman
Toomas Soome
Review request changed

Status: Closed (submitted)