some fixes for auto_bignum stuff

Rev: lib/master.pike.in:1.59
Rev: src/bignum.c:1.2
Rev: src/bignum.h:1.3
Rev: src/builtin_functions.c:1.184
Rev: src/language.yacc:1.125
Rev: src/main.c:1.77
Rev: src/operators.c:1.62
Rev: src/program.c:1.158