Userland: Add LibSystem and funnel all syscalls through it

This achieves two things:

- Programs can now intentionally perform arbitrary syscalls by calling
  syscall(). This allows us to work on things like syscall fuzzing.

- It restricts the ability of userspace to make syscalls to a single
  4KB page of code. In order to call the kernel directly, an attacker
  must now locate this page and call through it.

Userland/Utilities/crash.cpp

@@ -27,13 +27,13 @@
 
 #include <AK/Function.h>
 #include <AK/String.h>
-#include <Kernel/API/Syscall.h>
 #include <Kernel/IO.h>
 #include <LibCore/ArgsParser.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <sys/mman.h>
 #include <sys/wait.h>
+#include <syscall.h>
 
 #pragma GCC optimize("O0")
 

Userland/Utilities/functrace.cpp

@@ -32,7 +32,6 @@
 #include <AK/NonnullOwnPtr.h>
 #include <AK/StringBuilder.h>
 #include <AK/kmalloc.h>
-#include <Kernel/API/Syscall.h>
 #include <LibC/sys/arch/i386/regs.h>
 #include <LibCore/ArgsParser.h>
 #include <LibCore/File.h>
@@ -45,6 +44,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <syscall.h>
 #include <unistd.h>
 
 static OwnPtr<Debug::DebugSession> g_debug_session;

Userland/Utilities/purge.cpp

@@ -24,7 +24,6 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <Kernel/API/Syscall.h>
 #include <LibCore/ArgsParser.h>
 #include <serenity.h>
 #include <stdio.h>

Userland/Utilities/strace.cpp

@@ -27,7 +27,6 @@
 #include <AK/Assertions.h>
 #include <AK/LogStream.h>
 #include <AK/Types.h>
-#include <Kernel/API/Syscall.h>
 #include <LibC/sys/arch/i386/regs.h>
 #include <LibCore/ArgsParser.h>
 #include <LibCore/File.h>
@@ -37,6 +36,7 @@
 #include <string.h>
 #include <sys/ptrace.h>
 #include <sys/wait.h>
+#include <syscall.h>
 #include <unistd.h>
 
 static int g_pid = -1;

Userland/Utilities/syscall.cpp

@@ -24,13 +24,13 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
-#include <Kernel/API/Syscall.h>
 #include <errno.h>
 #include <getopt.h>
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <syscall.h>
 #include <unistd.h>
 
 #if !defined __ENUMERATE_SYSCALL