os-posix: use global daemon_pipe instead of cryptic fds[1]

os-posix: use global daemon_pipe instead of cryptic fds[1]

When asked to -daemonize, we fork a child and setup a pipe between
it and parent to pass exit status.  os-posix.c used global fds[2]
array for that, but actually only the writing side of the pipe is
needed to be global, and this name is really too generic.  Use
just one interger for the writing side of the pipe, and name it
daemon_pipe to be more understandable than cryptic fds[1].

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Gonglei <arei.gonglei@huawei.com>