Improve parsing of version strings

Improve parsing of version strings

* lisp/subr.el (version-regexp-alist): Allow "." as priority separator
(version-to-list): More helpful error messages.
(version-to-list): ".5" is valid (update docstring).  Make
"22.8X3" invalid, as the doc string says.

* test/automated/subr-tests.el (ert-test-version-parsing): New
tests for version string processing.

Copyright-paperwork-exempt: yes