Added a lot of missing PMOD_EXPORT.

Rev: src/fd_control.h:1.6
Rev: src/gc.h:1.73
Rev: src/main.h:1.15
Rev: src/mapping.h:1.33
Rev: src/multiset.h:1.16
Rev: src/object.h:1.56
Rev: src/opcodes.h:1.9
Rev: src/operators.h:1.9
Rev: src/pike_macros.h:1.23
Rev: src/pike_types.h:1.46
Rev: src/port.h:1.32
Rev: src/signal_handler.h:1.11
Rev: src/threads.h:1.109