win32: add mmap() emulation

Makes all of overlay_add work on windows/mingw.

Since we now don't explicitly check for mmap() anymore (it's always
present), this also requires us to make af_export.c compile, but I
haven't tested it.
command: overlay_add: more flexible treatment of offset parameter

Essentially, don't make it the mmap() argument, and just add it to the
memory address. This hides tricky things like alignment reequirements
from the user.

Strictly speaking, this is not entirely backwards compatible: this adds
the regression that you can't access past 2 or 4 GB of a file on 32 bit
systems anymore. But I doubt anyone cared about this.

In theory, we could be clever, and just align the offset manually and
pass that to mmap(). This would also be transparent to the user, but
minimally more effort, so this is left as exercise to the reader.