maktaba#syscall# doesn't work properly for Windows

The syscall support in maktaba tries to force `/bin/sh` when it checks if the shell value is usable, and doesn't work properly on Windows. Configuring it to allow `cmd` might help, but there may be other assumptions on what syntax the shell supports that won't work properly with Windows' cmd program.

When users are running into this issue, the symptom is that syscalls are failing with `E484: Can't open file C:\Users\\AppData\Local\Temp\VIoFBAB.tmp`.