mwrap 2.3.0

This release now supports --enable-shared builds of Ruby which
is the common setting for GNU/Linux distros.  FreeBSD 12.3
support remains broken, but the code is somewhat revised so it
can be more easily fixed and maintained in the future.

`mwrap --help' and and `mwrap --version' are now supported,
and an RDoc error which only happens during `gem install'
is fixed.

* support --enabled-shared builds of Ruby
* extconf.rb: avoid RDoc errors during gem install
* add --version and --help args
* cleanup some FreeBSD-related workarounds