Add Windows support

[fingolfin]

A user tried to use the GAP package kbmag on Windows -- of course we say that this is not supported, but perhaps somebody is interested in fixing that. The relevant error message they got was this:

Error, PrintTo: cannot open '/tmp/gaptempfile.lKvKhT' for output in
 CallFuncList( PRINT_TO, arg ); at /proc/cygdrive/C/gap-4.11.0/lib/streams.gd:1050 called from
PrintTo( filename, "_RWS := rec (\n" ); at /proc/cygdrive/C/gap-4.11.0/pkg/kbmag-1.5.9/gap/rws4.g:745 called from
WriteRWS( rws, _KBTmpFileName ); at /proc/cygdrive/C/gap-4.11.0/pkg/kbmag-1.5.9/gap/rws4.g:1529 called from
KBRWS( rws ) at /proc/cygdrive/C/gap-4.11.0/pkg/kbmag-1.5.9/gap/kbsmg2.gi:224 called from
<function "KnuthBendix for a KBMAG rewriting system">( <arguments> )
called from read-eval loop at *stdin*:5

I think part of the fix will include that we stop using Exec and instead use Process, which will also allow us to (better) handle errors in the invoked executables.

[dimpase]

if kbmag is broken on Windows it should not be in a semibroken state in the Windows distro, while the latter does have all its executables etc.