Add a similar option to repoman's--without-mask to pcheck

[jmbsvicetto]

Zac recently added a very useful option to repoman "--without-mask" that makes repoman silence the warnings about masked packages. An example where this would be useful is when working with the ebuilds for kde-4.1, as all ebuilds >4.0 are p. masked in /usr/portage/profiles/package.mask, pcheck keeps complaining about nonsolvable deps. An interesting alternative would be the possibility to have a package.unmask file at the <repo>/profiles dir - so that we could unmask certain packages provided by the repo, without requiring the user to copy a file or to set the file in /etc/portage.