The PAL API previously allowed for returning more memory than asked for.
This was when the PAL performed the alignment work, now this is done in
large alloc, so removing from the PAL.
Pull out a generic POSIX PAL as a superclass for the Linux and generic
BSD PALs. Now we have FreeBSD and Linux adding OS-specific behaviour,
OpenBSD as a named subclass of the generic BSD PAL that doesn't add any
behaviour.
I believe a NetBSD PAL should now be identical to the OpenBSD one -
patches welcome if anyone wants to test one!
