std: `std.require` now matches last dot-delimited version number.

* specs/std_spec.yaml (require): Specify behaviours when a version
string contains more than one substring with dot-delimited digits.
* lib/std/base.lua (module_version): Anchor the version matching
pattern at the end of the string.
* lib/std.lua.in (require): Improve LDocs accordingly.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>