msgen: Convert charset=CHARSET to charset=ASCII.
Reported at <https://savannah.gnu.org/bugs/index.php?20923>.
* gettext-tools/src/msgen.c: Include msgl-ascii.h, c-strstr.h, xalloc.h.
(fill_header): New function, based on fill_header in msginit.c.
(main): Invoke it.
* gettext-tools/tests/msgen-1: Provide as input a POT file with charset=CHARSET,
and verify that the output has charset=ASCII.
