Use HW_PHYSMEM in get_total_system_memory_impl

An upcoming PR will change get_total_system_memory_impl to use HW_PHYSMEM instead of HW_USERMEM in the case where HW_PHYSMEM is defined but some other methods are not available for getting the total physical memory (one example: FreeBSD).

The code actually checks that HW_PHYSMEM is defined, and comments reference HW_PHYSMEM, but HW_USERMEM is simply used instead.

For OpenBSD, NetBSD and OSX a 64-bit variant of HW_PHYSMEM is used when available.

The same PR will also contain a commit to update/fix a couple of comments in this same file. It will fix a typo in a comment and update another comment which suggests that HW_PHYSMEM64 is something only on OpenBSD when NetBSD actually defines it as well.

Trac:
Username: kjak