Add version numbers to the documentation

Add the version number at which each module option was added with its
current meaning to the documentatation.  Also improve the wording in
one place, and mention the Red Hat PAM module.
Add the no_update_user option

Add a no_update_user option that disables the normal update of the
PAM_USER PAM variable after canonicalization of the username.  When
this is set, pam-krb5 will not convert full principal names to local
usernames where possible for the rest of the PAM stack.
Add tests for POD documentation, fix spelling

Import the tests for POD syntax and documentation from the current
Git version of C TAP Harness.  Add lots of stopwords to the
documentation and fix a few spelling errors.