Attempt to display a manual page for plugins

Try do display the pkg-plugin man page.  If it's not available, this
result in:

~~~
% pkg help plugin
No manual entry for pkg-plugin
~~~

which is still better than:

~~~
% pkg help plugin
pkg: 'plugin' is not a valid command.

See 'pkg help' for more information on the commands.
~~~

if such a plugin is available...