flock: make flock(1) work on NFSv4

To pleace an exclusive lock on a file, NFSv4 requires
the file to be opened RW because of the emulation of flock()
by fcntl():

http://www.spinics.net/lists/linux-nfs/msg18502.html



So instead of O_RDONLY, open the file in O_RDWR if access()
indicates it is possible (unless shared lock is requested).

From: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Petr Uzel <petr.uzel@suse.cz>