fds: Introduce pipe_cloexec()

fds: Introduce pipe_cloexec()

Replace all calls to pipe() with this new function, which checks
errors for us, and also sets both fds to close-on-exec.

There are some minor functional changes:
 * Error messages from pipe() failing are now less detailed about the
   context.  This is not important.
 * The signal self-pipe is now cloexec too.  This is at worst harmless.
 * When execing userv-ipif we rely on cloexec to close the spare
   copies of the pipe ends.
 * The stderr self-pipe spare writing end is redudantly made cloexec
   even though it is about to be closed shortly afterwards.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>