jail: Fix an O(n^2) loop when adding jails

When a jail is added using the default (system-chosen) JID, and
non-default-JID jails already exist, a loop through the allprison
list could restart and result in unnecessary O(n^2) behaviour.
There should never be more than two list passes required.

Also clean up inefficient (though still O(n)) allprison list traversal
when finding jails by ID, or when adding jails in the common case of
all default JIDs.