New commands: mknod, mkfifo, mknod_b, mknod_c and umask.

New commands: mknod, mkfifo, mknod_b, mknod_c and umask.

These commands are used to create block and char device
nodes or FIFOs (named pipes) in the filesystem.

The umask command is required also because the permissions
used by mknod are masked by the umask.

Also document and guarantee that the umask starts as 022.