busybox: lock: implement -n "Fail rather than wait"

busybox: lock: implement -n "Fail rather than wait"

lock -n is similiar to flock -n. If the lock was already taken,
fail with exit code = 1 and write error message to stderr.

example:
if ! lock -n /tmp/foo ; then
	echo lock exits.
else
	echo lock was free. But is locked now.
fi
> lock was free. But is locked now.
> lock exists.

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@46836 3c298f89-4303-0410-b956-a3cf2f4a3e73