use common.regsub_file() instead of Popen(sed)

use common.regsub_file() instead of Popen(sed)

Python libraries work better in Python than running external commands, and
it also makes the code much more portable.  For example, the GNU and BSD
sed commands have different, and sometimes conflicting, flags.

This also reworks the regexp patterns to be more tightly focused, and not
change the same variable name in comments or elsewhere.