repository: verify the URL obeys the protocol

If enough time has passed since the URL was last retrieved (30 days by
default), try to access it using a method that depends on the
protocol. The retrieved qualifier is updated if it succeeds, otherwise
an error is displayed.

Change-Id: Ib01897068ba4a492a7ea2d3cec89570240ea0a1a
Signed-off-by: Loic Dachary <loic@dachary.org>