rpki-client: rework AIA, SIA, and CRL handling

rpki-client: rework AIA, SIA, and CRL handling

Assert that the out parameter is NULL rather than blindly NULL-ing it,
and follow the approach in sbgp_sia() more closely: use a local variable
and warn on any ignored accessMethod/distributionPoint in verbose mode.
AIA is slightly different because there's only a single accessMethod.

Drop the now useless (and previously wrong) warning on *out != NULL in
x509_location(). Instead, assert that the out parameter is NULL. This
way things are a bit less clever, but more explicit, more correct, more
robust, and the code grew only very slightly.

ok claudio