Regenerate ctlseqs.txt from ctlseqs.ms

This requires groff, so add it to Build-Depends.

It would have been nicer to just remove ctlseqs.txt in the clean
target, but the missing files means a tree dirty when building from
git.  And removing it from the debian-unstable branch would cause
conflicts when merging future upstream releases.  So I back up the
file before the build and restore it in the clean target.