added reenter check to lock_dequeue() to avoid unlocking of interrupts

added reenter check to lock_dequeue() to avoid unlocking of interrupts
via cause_sig() during an exception.

moved lock check configuration to <minix/sys_config.h> instead of
kernel/config.h, because the 'relocking' field in kinfo depends on it.

other prettification: common locking macro, whether lock timing is on or
not.