Implement basic sys$kill() and add a /bin/kill

All it can do right now is send SIGKILL which just murders the target task.

Userland/kill.cpp

@@ -0,0 +1,37 @@
+#include <LibC/unistd.h>
+#include <LibC/stdio.h>
+#include <LibC/signal.h>
+#include <AK/String.h>
+
+static unsigned parseUInt(const String& str, bool& ok)
+{
+    unsigned value = 0;
+    for (size_t i = 0; i < str.length(); ++i) {
+        if (str[i] < '0' || str[i] > '9') {
+            ok = false;
+            return 0;
+        }
+        value = value * 10;
+        value += str[i] - '0';
+    }
+    ok = true;
+    return value;
+}
+
+int main(int argc, char** argv)
+{
+    if (argc < 2) {
+        printf("usage: kill <PID>\n");
+        return 1;
+    }
+    bool ok;
+    unsigned value = parseUInt(argv[1], ok);
+    if (!ok) {
+        printf("%s is not a valid PID\n", argv[1]);
+        return 2;
+    }
+
+    kill((pid_t)value, SIGKILL);
+    return 0;
+}
+