Better debug checks on the interpreter lock.

Rev: src/interpret.c:1.144
Rev: src/interpreter.h:1.41
Rev: src/modules/HTTPLoop/accept_and_parse.c:1.12
Rev: src/modules/HTTPLoop/cache.c:1.9
Rev: src/modules/Java/jvm.c:1.15
Rev: src/modules/files/file.c:1.174
Rev: src/modules/files/sendfile.c:1.38
Rev: src/program.c:1.229
Rev: src/signal_handler.c:1.169
Rev: src/threads.c:1.122
Rev: src/threads.h:1.86