Kernel+Userland: Implement mknod() syscall and add a /bin/mknod program.

Userland/mknod.cpp

@@ -0,0 +1,50 @@
+#include <unistd.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/stat.h>
+
+inline constexpr unsigned encoded_device(unsigned major, unsigned minor)
+{
+    return (minor & 0xff) | (major << 8) | ((minor & ~0xff) << 12);
+}
+
+static int usage()
+{
+    printf("usage: mknod <name> <c|b|p> <major> <minor>\n");
+    return 0;
+}
+
+int main(int argc, char** argv)
+{
+    // FIXME: When invoked with type "p", no need for major/minor numbers.
+    // FIXME: Add some kind of option for specifying the file permissions.
+    if (argc != 5)
+        return usage();
+
+    const char* name = argv[1];
+    mode_t mode = 0666;
+    switch (argv[2][0]) {
+    case 'c':
+    case 'u':
+        mode |= S_IFCHR;
+        break;
+    case 'b':
+        mode |= S_IFBLK;
+        break;
+    case 'p':
+        mode |= S_IFIFO;
+        break;
+    default:
+        return usage();
+    }
+
+    int major = atoi(argv[3]);
+    int minor = atoi(argv[4]);
+
+    int rc = mknod(name, mode, encoded_device(major, minor));
+    if (rc < 0) {
+        perror("mknod");
+        return 1;
+    }
+    return 0;
+}