Use execprog() instead of system() ... partly transition

Use execprog() instead of system() ... partly transition

There are several more places where system() should be replaced.

As execprog() does not split strings into multiple arguments,
`listproc' could no longer contain ``show -file''. We need to
rework the concept of these `...proc' entries.

This commit fixes tests/whatnow/test-attach-detach.