add VMM functions, prepare some DDocs

So far only Win32 stuff is tested
add vmm.d to makefiles
Tweak DDocs as suggested by the review
reintroduce Posix version
Another pass through VMM module

Remove VMM struct.
Few attempts to fix posix version.
Add missing constants to sys.posix.unistd.

Some tweaks to DDocs
OS page size --> platform's page size.

Linux and Windows now are forced to discard pages on decommit,
unlike POSIX madvise which is overly lax on guarantees.
add more constants from BSD headers
Forget sysconf, use unistd.h getpagesize call on *NIX
remove SC constants, no longer need these
restore ANON for other posix