Add support for nommu systems.

Add support for nommu systems.

Rather than hardcode the WIN32 define, add proper fork checks to the
configure script and check those.  This fixes building for nommu systems
which lack the fork function.

While we're here though, add support for this functionality via vfork
so that it does work on nommu systems.  And fix an old bug where we
exit properly in the forked child when the exec failed instead of just
returning to the calling code (which isn't expecting it).

Reviewed-By: Guy Harris <[email protected]>