Only _WIN32 is defined by the compiler, and WIN32 is not standardized
and may be missing.

Closes #169

Enforcing MADV_DONTFORK is a surprising limitation for this library
which aims to be generic.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Just in case.

There were two ways this could fail:

1. division by zero when sysconf(_SC_PAGESIZE)==0

2. mmap() failure because the size parameter is not aligned to page
   size

Neither ever happened: sysconf() never fails, and the only caller
passes a size that is already aligned.  Phew.