gpg: --compliance=gnupg restores default for policy-affected options

* g10/gpg.c (set_compliance_option): oGnuPG restores default
policy-affected options, moved from...
(main): ...here.  Invoke set_compliance_option(oGnuPG) directly
instead of just setting opt.compliance.

Some of these default option values (flags.dsa2, rfc2440_text,
allow_non_selfsigned_uid, allow_freeform_uid) had to be inferrerd from
the fact that the opt struct is static and therefore initialized to
zero by the compiler.

With this change, --compliance=gnupg now completely reverts to the
defaults that were changed from other --compliance= options.

• GnuPG-bug-id: T7501
• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>