gpgscm: Improve syntax dispatch.
* tests/gpgscm/scheme.c (assign_syntax): Add opcode parameter, store opcode in the tag. (syntaxnum): Add sc parameter, retrieve opcode from tag. (opexe_0): Adapt callsite. (scheme_init_custom_alloc): Likewise.
- Signed-off-by: Justus Winter <justus@g10code.com>