Improved _disable_threads().

Added THREADS_ALLOW_UID() and THREADS_DISALLOW_UID().

Rev: src/modules/files/efuns.c:1.47
Rev: src/modules/files/file.c:1.83
Rev: src/modules/files/socket.c:1.30
Rev: src/modules/spider/dumudp.c:1.36
Rev: src/modules/system/passwords.c:1.7
Rev: src/modules/system/syslog.c:1.5
Rev: src/modules/system/system.c:1.47
Rev: src/program.c:1.67
Rev: src/threads.c:1.65
Rev: src/threads.h:1.37