callbacks modified to handle recursive calls better

Rev: src/backend.c:1.7
Rev: src/builtin_functions.c:1.15
Rev: src/callback.c:1.3
Rev: src/callback.h:1.2
Rev: src/gc.h:1.5
Rev: src/interpret.c:1.15
Rev: src/interpret.h:1.5
Rev: src/main.c:1.12