PO mode: Fix recognition of C and C++ modes.
The variable 'mode-name' can contain a suffix, see
https://www.gnu.org/software/emacs/manual/html_node/ccmode/Minor-Modes.html
https://www.gnu.org/software/emacs/manual/html_node/emacs/Electric-C.html
Therefore use 'major-mode' instead of 'mode-name'.
* gettext-tools/misc/po-mode.el (po-preset-string-functions): Test major-mode,
not mode-name.
(po-mode-version-string): Bump to 2.25.
Reported at <https://savannah.gnu.org/bugs/index.php?50830>
by Peter Hull <peterhull90@gmail.com>.
