Restricted-to mirrors are not checked for freshness

[rgeissert]

Commits 1e33f17 and 306b6d3 (part of issue #3) introduced support for the Restricted-to field in the master list. However, due to check.pl's per-continent age check and the way the restriction was implemented (by simply not adding the mirrors to some indexes), they are not checked for freshness.

This could lead to inconsistencies that would only be noticed in the scope of the mirror's restriction (AS, country, etc.)