Various f_cast()-related fixes.

Rev: src/interpret_functions.h:1.47
Rev: src/module_support.c:1.40
Rev: src/modules/Gmp/mpz_glue.c:1.88
Rev: src/modules/files/file.c:1.212
Rev: src/opcodes.c:1.100
Rev: src/operators.c:1.124