flock does not work on AIX

[Taywee]

flock doesn't work on AIX because it doesn't actually reside in libc, but instead in libbsd. flock is necessary for full functionality of jruby, though. Some obvious solutions:

1. Create an interface to libbsd, load it in, and call flock there instead.
2. Override flock in the AIX interface, and translate arguments and call either lockf or fcntl with the equivalent arguments instead (this should work in AIX because lockf, flock, and fcntl locks are integrated and are enforced across all the APIs), essentially reimplementing flock in terms of lockf/fcntl.

I feel the latter is probably the easiest and least disruptive option, but it will require the extra struct flock type and some extra interfaces. The fcntl path is probably the easiest to avoid having to deal with things like AIX's lockf and lockf64, as well as simply being more flexible in general.