Scan OPS files directly for gettext translation strings.

Previously, gettext's po/Makefile.in.in was modified to separately
scan and merge a generated file: keymap_alldefs.h.  Gettext doesn't
recommend nor provide support for scanning generated files.

However, there is no reason we have to scan the generated file.
Instead move the N_() markers and L10N comments into the OPS files.
Then simply add the OPS files to po/POTFILES.in.

Add a '--language=C' option to xgettext to avoid a warning about the
OPS files not have any known language suffix.

Modify gen_defs and doc/gen-map-doc to deal with the new format of the
OPS files.