Retry short writes when flushing WAL.

We don't normally bother retrying when the number of bytes written by
write() is short of what was requested. It is generally assumed that a
write() to disk doesn't return short, unless you run out of disk space.
While writing the WAL, however, it seems prudent to try a bit harder,
because a failure leads to PANIC. The write() is also much larger than most
write()s in the backend (up to wal_buffers), so there's more room for
surprises.

Also retry on EINTR. All signals used in the backend are flagged SA_RESTART
nowadays, so it shouldn't happen, but better to be defensive.