Fix hang in `named-compilezone | head`

Fix hang in `named-compilezone | head`

I was truncating zone files for experimental purposes when I found
that `named-compilezone | head` got stuck. The full command line that
exhibited the problem was:

	dig axfr dotat.at |
	named-compilezone -o /dev/stdout dotat.at /dev/stdin |
	head

This requires a large enough zone to exhibit the problem, more than
about 70000 bytes of plain text output from named-compilezone.
I was running the command on Debian Stretch amd64.

This was puzzling since it looked like something was suppressing the
SIGPIPE. I used `strace` to examine what was happening at the hang.
The program was just calling write() a lot to print the zone file, and
the last write() hanged until I sent it a SIGINT.

During some discussion with friends, Ian Jackson guessed that opening
/dev/stdout O_RDRW might be the problem, and after some tests we found
that this does in fact suppress SIGPIPE.

Since `named-compilezone` only needs to write to its output file, the
fix is to omit the stdio "+" update flag.