Change in internal timehandling; ability to invalidate current_time.

src/backend.cmod

src/backend.h

src/builtin.cmod

src/builtin_functions.c

src/modules/files/file.c

src/modules/files/socket.c

src/modules/files/udp.c

src/modules/system/system.c

src/pike_embed.c

src/port.c

src/post_modules/GTK1/source/global.pre

src/post_modules/GTK2/source/global.pre

src/program.c

src/rusage.c

src/signal_handler.c

src/threads.c

src/time_stuff.h