parse-datetime: better name than get_date

parse-datetime: better name than get_date

* NEWS: Reword the deprecation notice.
* modules/get_date: Rename to modules/parse-datetime.
* modules/get_date-tests: Rename to modules/parse-datetime-tests.
* m4/get_date.m4: Rename to m4/parse-datetime.m4.
* lib/get_date.y: Rename to lib/parse-datetime.y.
* tests/test-get_date.c: Rename to tests/test-parse-datetime.c.
* doc/get_date.texi: Rename to doc/parse-datetime.texi.
* doc/getdate.texi: Provide fallback wrapper.
* lib/getdate.h: Move guts, and wrap...
* lib/parse-datetime.h: ...new file.
* lib/parse-datetime.y (get_date): Rename...
(parse_datetime): ...to this.
* m4/parse-datetime.m4 (gl_GET_DATE): Rename...
(gl_PARSE_DATETIME): ...to this.
* doc/posix-functions/getdate.texi (get_date): Provide fallback
documentation.
* modules/getdate (Files): Provide fallback docs and header.
(Notice, Depends-on): Update references.
* tests/test-parse-datetime.c: Likewise.
* DEPENDENCIES: Likewise.
* MODULES.html.sh (Date and time <time.h>): Likewise.
* doc/parse-datetime.texi (Date input formats)
(Authors of parse_datetime): Likewise.
* modules/parse-datetime (Files, configure.ac, Makefile.am)
(Include): Likewise.
* modules/parse-datetime-tests (Files, Makefile.am): Likewise.
* gnulib-tool: Likewise.
* m4/bison.m4 (gl_BISON): Likewise.
Suggested by Bruno Haible.

Signed-off-by: Eric Blake <eblake@redhat.com>