getdate: rename to get_date

getdate: rename to get_date

Note: getdate.h is not renamed, to minimize client impact.
Besides, whenever we eventually add getdate-posix, we'll
want to use getdate.m4 and getdate.c for the implementation,
but don't need to worry about getdate.h since getdate() uses
<time.h> for its header.

* modules/getdate: Mark obsolete.  Move old contents...
* modules/get_date: ...to new module name.
* modules/getdate-tests: Move...
* modules/get_date-tests: ...here.
* m4/getdate.m4: Move...
* m4/get_date.m4: ...here, and rename gl_GETDATE to gl_GET_DATE.
* lib/getdate.y: Move...
* lib/get_date.y: ...here.
* tests/test-getdate.c: Move...
* tests/test-get_date.c: ...here.
* doc/posix-functions/getdate.texi (getdate): Update name.
* NEWS: Mention the change.

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