Refactor.
* gettext-tools/src/msgl-header.h (header_set_charset): New declaration.
* gettext-tools/src/msgl-header.c (header_set_charset): New function.
* gettext-tools/src/msgl-iconv.c: Include msgl-header.h.
(iconv_message_list_internal): Use header_set_charset.
* gettext-tools/src/x-po.c: Include msgl-header.h.
(extract): Use header_set_charset.
* gettext-tools/src/FILES: Update.
