libstand: Fix IP recv timeout
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.