Userland: Add syscall program

The Plan9 OS has this  program that can test a system call with the
given arguments. For the most basic system calls it can be very
helpful and aid with testing or just to play with a given syscall
without writing a dedicated program.

Some examples:
    syscall write 1 hello 5
    syscall -o read 0 buf 5
    syscall mkdir /tmp/my-dir
    syscall exit 2
    ...

Userland/syscall.cpp

@@ -0,0 +1,88 @@
+#include <Kernel/Syscall.h>
+#include <errno.h>
+#include <getopt.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+
+#if !defined __ENUMERATE_SYSCALL
+#    define __ENUMERATE_SYSCALL(x) SC_##x,
+#endif
+
+#define SC_NARG 4
+
+Syscall::Function syscall_table[] = {
+    ENUMERATE_SYSCALLS
+};
+
+uintptr_t arg[SC_NARG];
+char buf[BUFSIZ];
+
+uintptr_t parse(char* s);
+
+int main(int argc, char** argv)
+{
+    int oflag;
+    int opt;
+    while ((opt = getopt(argc, argv, "oh")) != -1) {
+        switch (opt) {
+        case 'o':
+            oflag = 1;
+            break;
+        case 'h':
+            fprintf(stderr, "usage: \tsyscall [-o] entry [args; buf==BUFSIZ buffer]\n");
+            fprintf(stderr, "\tsyscall write 1 hello 5\n");
+            fprintf(stderr, "\tsyscall -o read 0 buf 5\n");
+            fprintf(stderr, "\tsyscall -o getcwd buf 100\n");
+            break;
+        default:
+            exit(EXIT_FAILURE);
+        }
+    }
+
+    if (optind >= argc) {
+        fprintf(stderr, "No entry specified\n");
+        return -1;
+    }
+
+    for (int i = 0; i < argc - optind; i++) {
+        arg[i] = parse(argv[i + optind]);
+    }
+
+    for (auto sc : syscall_table) {
+        if (strcmp(Syscall::to_string(sc), (char*)arg[0]) == 0) {
+            int rc = syscall(sc, arg[1], arg[2], arg[3]);
+            if (rc == -1) {
+                perror("syscall");
+            } else {
+                if (oflag)
+                    printf("%s", buf);
+            }
+
+            fprintf(stderr, "Syscall return: %d\n", rc);
+            return 0;
+        }
+    }
+
+    fprintf(stderr, "Invalid syscall entry %s\n", (char*)arg[0]);
+    return -1;
+}
+
+uintptr_t parse(char* s)
+{
+    char* t;
+    uintptr_t l;
+
+    if (strcmp(s, "buf") == 0) {
+        return (uintptr_t)buf;
+    }
+
+    l = strtoul(s, &t, 0);
+    if (t > s && *t == 0) {
+        return l;
+    }
+
+    return (uintptr_t)s;
+}