pinspect: add stable_{forced,masked}_use subcmds for profiles

Allows users to easily see the difference between stable and unstable
profile usage.

Note that it doesn't make sense to automatically determine which to show
since that relates to a pkg's KEYWORDS value and the ACCEPT_KEYWORDS
setting for a local machine's configuration.
pebuild: allow atom or ebuild file arguments

Now you can also pass an ebuild file directly to pebuild to operate on
(as you might be used to from using portage's ebuild command). Note that
it is currently required for the ebuild to be in a configured
repository.

Fixes #14.