Refactor: Librarify the core of msgmerge.

Refactor: Librarify the core of msgmerge.

* gettext-tools/src/msgl-merge.h: New file, based on
gettext-tools/src/msgmerge.c.
* gettext-tools/src/msgl-merge.c: New file, based on
gettext-tools/src/msgmerge.c.
* gettext-tools/src/msgmerge.c: Include msgl-merge.h. Simplify includes.
Remove definitions that are moved to msgl-merge.c.
* gettext-tools/src/FILES: Update.
* gettext-tools/src/Makefile.am (noinst_HEADERS): Add msgl-merge.h.
(msgmerge_SOURCES): Add msgl-merge.c.
* gettext-tools/po/POTFILES.in: Add src/msgl-merge.c.