Kernel: Userspace signal handlers are now called one at a time

I added a syscall for telling the kernel when signal execution has
finished. We should send a random hash or id to the signal trampoline
that we would include in the syscall, so validity of signal exit can
be confirmed.

libc/include/sys/syscall.h

@@ -44,6 +44,7 @@ __BEGIN_DECLS
 #define SYS_RAISE 37
 #define SYS_KILL 38
 #define SYS_SIGNAL 39
+#define SYS_SIGNAL_DONE 40
 
 __END_DECLS
 

libc/unistd.cpp

@@ -286,6 +286,9 @@ long syscall(long syscall, ...)
 			ret = Kernel::syscall(SYS_SIGNAL, signal, (uintptr_t)handler);
 			break;
 		}
+		case SYS_SIGNAL_DONE:
+			// Should not be called by an user
+			// fall through
 		default:
 			puts("LibC: Unhandeled syscall");
 			ret = -ENOSYS;