gpgsm: Always use the chain model if the root-CA requests this.

* sm/call-dirmngr.c (gpgsm_dirmngr_isvalid): Do not use
option --force-default-responder.
* sm/certchain.c (is_cert_still_valid): Rename arg for clarity.
(gpgsm_validate_chain): Always switch to chain model.

The trustlist.txt may indicate that a root CA issues certificates
which shall be validated using the chain model. This is for example
the case for qualified signatures. Before this change we did this
only if the default shell model indicated that a certificate has
expired. This optimization is technically okay but has one problem:
The chain model requires the use of OCSP but we switch to this only
when running the chain model validation. To catch revoked
certificates using OCSP we need to always switch to the chain model
unless OCSP has been enabled anyway.

Note that the old --force-default-responder option is not anymore
used.

Test cases are certificates issued by

1. CN=TeleSec qualified Root CA 1
2. O=Deutsche Telekom AG
3. C=DE
4. 2.5.4.97=USt-IdNr. DE 123475223 90:C6:13:6C:7D:EF:EF:E9:7C:C7:64:F9:D2:67:8E:AD:03:E5:52:96 \ S cm qual relax

A sample revoked certificate is

-----BEGIN CERTIFICATE-----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-----END CERTIFICATE-----