Rename some files.
* gettext-tools/src/read-po-internal.h: Renamed from
gettext-tools/src/po-gram.h.
* gettext-tools/src/read-po-lex.h: Renamed from gettext-tools/src/po-lex.h.
* gettext-tools/src/read-po-lex.c: Renamed from gettext-tools/src/po-lex.c.
* gettext-tools/src/read-po-gram.y: Renamed from
gettext-tools/src/po-gram-gen.y.
* gettext-tools/src/*.h: Update.
* gettext-tools/src/*.c: Update.
* gettext-tools/src/FILES: Update.
* gettext-tools/src/Makefile.am: Update.
* gettext-tools/libgettextpo/Makefile.am: Update.
