Cygwin: fcntl: implement Open File Description (OFD) locks

Partially quoting the LINUX man page:

Open file description locks are advisory byte-range locks whose opera‐
tion is in most respects identical to the traditional record locks de‐
scribed above.

The principal difference between the two lock types is that whereas tra‐
ditional record locks are associated with a process, open file descrip‐
tion locks are associated with the open file description on which they
are acquired, much like locks acquired with flock(2). Consequently (and
unlike traditional advisory record locks), open file description locks
are inherited across fork(2) (and clone(2) with CLONE_FILES), and are
only automatically released on the last close of the open file descrip‐
tion, instead of being released on any close of the file.

Conflicting lock combinations (i.e., a read lock and a write lock or two
write locks) where one lock is an open file description lock and the
other is a traditional record lock conflict even when they are acquired
by the same process on the same file descriptor.

Just like traditional advisory record locks, OFD locks do not conflict
with BSD file locks acquired with flock(2).

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

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