OpenBSD support.

[Ori Bernstein <ori@xxxxxxxxxxxxxx>]

Everything but threads is working, so I pushed it.

Changes needed were relatively minimal, as expected for
a BSD. In the future, it might be nice to have something
that trawls syscalls.master and /usr/include to generate
the calls programmatically, since manually translating
the numbers and structs is rather miserable work.

One other thing to note: I needed to align mmap sizes
to page size in syswrap+posixy.myr or we would trash
some data.  I'm curious about why that is, and I'm
wondering if this is covering some other bug.

Other than that, have fun.

-- 
    Ori Bernstein