Add Porting/mksample

Add Porting/mksample

This script was previously in the metaconfig repository, and also dealt
with updating Porting/Glossary, but it wasn't being regularly used.
It does not use any data from that repository, so is better placed
here.

There are some outstanding ambiguities to resolve in when this script
should be run in the documentation, since the generated files
Porting/config.sh and Porting/config_H are also updated by other
means.