maint: straggler *.[ch] files: convert more `...' to '...'

maint: straggler *.[ch] files: convert more `...' to '...'

The preceding commands ignored .[ch] files in lib/ and gl/.
This is what I should have been doing from the start:

  git grep -l '`.*'\' $(g ls-files |grep '\.[ch]$') \
    | xargs perl -pi -e 's/`(.+?'\'')/'\''$1/'