mount.nfs: Replace fork() with daemon() for backgrounding mounts

Neil recommended this change to address a problem with background mount
processes handling signals properly during an init level change.  It is
also useful for preventing background mount processes from reporting
progress on the parent's tty, which is generally just annoying noise.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Neil Brown <neilb@suse.de>