garbage collector improved

Rev: src/gc.c:1.11
Rev: src/gc.h:1.9
Rev: src/interpret.c:1.43
Rev: src/modules/files/file.c:1.49
Rev: src/modules/files/socket.c:1.13
Rev: src/modules/files/testsuite.in:1.6
Rev: src/program.h:1.17
Rev: src/svalue.c:1.13
Rev: src/svalue.h:1.9