tools: flock: add NFSv4 compatibility

This patch fixes the LEDE build on mounted NFSv4 shares.

The lock file cannot be opened in read-write mode by default, because
then we cannot use flock(1) to lock executable files.

The read-write mode for lock files is necessary on NFSv4 where
flock(2) is emulated by by fcntl() -- this situation is possible to
detect by flock(2) EBADF error.

The patch consist of the following util-linux/flock commits

http://git.kernel.org/cgit/utils/util-linux/util-linux.git/commit/?id=eb742a1f66d5e3a7c5b43efce741c113f51bef3b

http://git.kernel.org/cgit/utils/util-linux/util-linux.git/commit/?id=caf1ba11a367ad702fb774653daf9ebdcca49d7b



without including the pre kernel 3.4 support.

Signed-off-by: Mathias Kresin <dev@kresin.me>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [minor cleanup]