gpgscm: Add flag TAIL_CONTEXT.
* tests/gpgscm/scheme.c (S_FLAG_TAIL_CONTEXT): New macro. This flag indicates that the interpreter is evaluating an expression in a tail context (see R5RS, section 3.5). (opexe_0): Clear and set the flag according to the rules layed out in R5RS, section 3.5. (opexe_1): Likewise.
- Signed-off-by: Justus Winter <justus@g10code.com>