Implement argc/argv support for spawned tasks.

Celebrate the new functionality with a simple /bin/cat implementation. :^)

Userland/cat.cpp

@@ -0,0 +1,30 @@
+#include <LibC/stdio.h>
+#include <LibC/unistd.h>
+#include <LibC/errno.h>
+#include <LibC/string.h>
+
+int main(int argc, char** argv)
+{
+    if (argc != 2) {
+        printf("usage: cat <file>\n");
+        return 1;
+    }
+    int fd = open(argv[1]);
+    if (fd == -1) {
+        printf("failed to open %s: %s\n", argv[1], strerror(errno));
+        return 1;
+    }
+    for (;;) {
+        char buf[128];
+        ssize_t nread = read(fd, buf, sizeof(buf));
+        if (nread == 0)
+            break;
+        if (nread < 0) {
+            printf("read() error: %s\n", strerror(errno));
+            return 2;
+        }
+        for (ssize_t i = 0; i < nread; ++i)
+            putchar(buf[i]);
+    }
+    return 0;
+}