Added Pfadvise and constant advise values. Untested.
Merge pull request #122 from cathalgarvey/luaposix

Entirely rewritten.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
Merge pull request 122 from cathalgarvey/luaposix

Almost entirely rewritten to be lighter, correctly formatted, and
to work when the host has no posix_advise implementation.