mk/subst.mk: refactor main code

The indentation of the inner loop has been fixed.

The chmod is only run if the file has actually changed. In the other
case, the file would have been removed right after the chmod, which made
the chmod unnecessary.

For compatibility with ancient operating systems whose /bin/sh still does
not understand negated conditions (SunOS), these conditions have been
avoided and were written using && and || instead.

The inner loop has been flattened a bit, to compensate for the
indentation of the outer loop.

(rillig)