add release test to ensure Jira tickets are in Changes

A release test will generate into xt/ that checks that any Jira ticket
metioned in the commit history since the last tag is also mentioned in
the latest section of the Changes file.
revert incomplete work on PERL-233
updated Changes in preparation for release
avoid generating boilerplate Pod for private packages
PERL-341 add installation instructions document
remove pseudo-pod directives from private packages
PERL-317 clarify documetation about thread support
add Changes entry for PERL-341