Support stripped secretkeys

This implements #570.

* adds a has_secret column to the keys table
* on import of secret keyrings, set availability of secret key for all subkeys
* displays "stripped" status in keyring view
* mark unavailable keys in selectsecretkeyactivity

Note that the database change is not production level quality! The has_secret flag will not be set for keys that were imported before the column is added. This won't matter for 2.5 -> 2.6 transition, but in the dev phase you'll have to re-import keys.