Kernel/LibC: Add bareboness signals

You can now call raise() to raise a signal. Signal handlers are
not yet supported, but the handling works :)

libc/include/signal.h

@@ -74,6 +74,9 @@ struct sigevent
 #define SIGRTMIN	29
 #define SIGRTMAX	(SIGRTMIN+32)
 
+#define _SIGMIN SIGABRT
+#define _SIGMAX SIGRTMAX
+
 #define SIG_BLOCK	1
 #define SIG_UNBLOCK	2
 #define SIG_SETMASK	3

libc/include/sys/syscall.h

@@ -41,6 +41,7 @@ __BEGIN_DECLS
 #define SYS_CLOCK_GETTIME 34
 #define SYS_PIPE 35
 #define SYS_DUP2 36
+#define SYS_RAISE 37
 
 __END_DECLS