fork() now returns an object thread_id->wait() implemented

Rev: lib/master.pike:1.43.2.1
Rev: lib/modules/Process.pmod:1.7.2.1
Rev: src/backend.c:1.10.2.1
Rev: src/backend.h:1.2.2.1
Rev: src/configure.in:1.93.2.1
Rev: src/language.yacc:1.42.2.1
Rev: src/mapping.c:1.18.2.1
Rev: src/modules/files/efuns.c:1.22.2.1
Rev: src/modules/files/file.c:1.37.2.1
Rev: src/modules/files/socktest.pike:1.2.2.1
Rev: src/modules/readline/readlinemod.c:1.2.2.1
Rev: src/object.c:1.17.2.1
Rev: src/object.h:1.9.2.1
Rev: src/program.c:1.33.2.1
Rev: src/program.h:1.14.2.1
Rev: src/signal_handler.c:1.11.2.1
Rev: src/threads.c:1.23.2.1