Kernel: Rework all signal code

Signal handling code was way too complex. Now everything is
simplified and there is no need for ThreadBlockers.

Only complication that this patch includes is that blocking syscalls
have to manually be made interruptable by signal. There might be some
clever solution to combat this is make this happen automatically.

libc/include/sys/syscall.h

@@ -40,7 +40,6 @@ __BEGIN_DECLS
 #define SYS_DUP2 37
 #define SYS_KILL 39
 #define SYS_SIGNAL 40
-#define SYS_SIGNAL_DONE 41
 #define SYS_TCSETPGRP 42
 #define SYS_GET_PID 43
 #define SYS_GET_PGID 44