xgettext: Share some more code among extractors.

xgettext: Share some more code among extractors.

* gettext-tools/src/xg-pos.h: New file.
* gettext-tools/src/xg-pos.c: New file.
* gettext-tools/src/Makefile.am (noinst_HEADERS): Add xg-pos.h.
(xgettext_SOURCES): Add xg-pos.c.
* gettext-tools/src/FILES: Update.
* gettext-tools/src/x-awk.c: Include xg-pos.h.
(real_file_name, logical_file_name, line_number): Remove variables.
* gettext-tools/src/x-c.c: Likewise.
* gettext-tools/src/x-csharp.c: Likewise.
* gettext-tools/src/x-elisp.c: Likewise.
* gettext-tools/src/x-java.c: Likewise.
* gettext-tools/src/x-javascript.c: Likewise.
* gettext-tools/src/x-librep.c: Likewise.
* gettext-tools/src/x-lisp.c: Likewise.
* gettext-tools/src/x-lua.c: Likewise.
* gettext-tools/src/x-perl.c: Likewise.
* gettext-tools/src/x-php.c: Likewise.
* gettext-tools/src/x-python.c: Likewise.
* gettext-tools/src/x-rst.c: Likewise.
* gettext-tools/src/x-scheme.c: Likewise.
* gettext-tools/src/x-sh.c: Likewise.
* gettext-tools/src/x-smalltalk.c: Likewise.
* gettext-tools/src/x-tcl.c: Likewise.
* gettext-tools/src/x-vala.c: Likewise.
* gettext-tools/src/x-ycp.c: Likewise.