gpgscm: Add 'finally', rework all macros.

* tests/gpgscm/init.scm (finally): New macro.
* tests/gpgscm/tests.scm (letfd): Rewrite.
(with-working-directory): Likewise.
(with-temporary-working-directory): Likewise.
(lettmp): Likewise.
--

Rewrite all our macros using 'define-macro'. Use the new control flow
mechanism 'finally', or 'dynamic-wind' where appropriate.  Make sure
the macros are hygienic.  Reduce code duplication.

Signed-off-by: Justus Winter <justus@g10code.com>

tests/gpgscm/init.scm

@@ -569,6 +569,16 @@
 ;    the thrown exception is bound to *error*.  Errors can be rethrown
 ;    using (rethrow *error*).
 ;
+;    Finalization can be expressed using "finally":
+;
+;         (finally (finalize-something called-purely-for side-effects)
+;              (whether-or-not something goes-wrong)
+;              (with-these calls))
+;
+;    The final expression is executed purely for its side-effects,
+;    both when the function exits successfully, and when an exception
+;    is thrown.
+;
 ;    Exceptions are thrown with:
 ;
 ;         (throw "message")
@@ -622,6 +632,13 @@
                     (pop-handler)
                     ,label)))))
 
+(define-macro (finally final-expression . expressions)
+  (let ((result (gensym)))
+    `(let ((,result (catch (begin ,final-expression (rethrow *error*))
+			   ,@expressions)))
+       ,final-expression
+       ,result)))
+
 ;; Make the vm use throw'.
 (define *error-hook* throw')