Kernel+LibC: Move LibC/signal_numbers.h to Kernel/API/POSIX

Make Userland and Tests users just include signal.h, and move Kernel
users to the new API file.

Kernel/API/POSIX/signal_numbers.h

@@ -0,0 +1,42 @@
+/*
+ * Copyright (c) 2018-2020, Andreas Kling <kling@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#define SIGINVAL 0
+#define SIGHUP 1
+#define SIGINT 2
+#define SIGQUIT 3
+#define SIGILL 4
+#define SIGTRAP 5
+#define SIGABRT 6
+#define SIGBUS 7
+#define SIGFPE 8
+#define SIGKILL 9
+#define SIGUSR1 10
+#define SIGSEGV 11
+#define SIGUSR2 12
+#define SIGPIPE 13
+#define SIGALRM 14
+#define SIGTERM 15
+#define SIGSTKFLT 16
+#define SIGCHLD 17
+#define SIGCONT 18
+#define SIGSTOP 19
+#define SIGTSTP 20
+#define SIGTTIN 21
+#define SIGTTOU 22
+#define SIGURG 23
+#define SIGXCPU 24
+#define SIGXFSZ 25
+#define SIGVTALRM 26
+#define SIGPROF 27
+#define SIGWINCH 28
+#define SIGIO 29
+#define SIGINFO 30
+#define SIGSYS 31
+#define SIGCANCEL 32
+#define NSIG 33