Change mknod test to use Unix::Mknod and POSIX::mkfifo().

Change mknod test to use Unix::Mknod and POSIX::mkfifo().

Instead of using system() to call executables (and depend on mknod
and mkfifo command syntax), use Unix::Mknod's mknod() function
and POSIX's mkfifo() function to make device and named pipe nodes,
and use Unix::Mknod's makedev() to combine major/minor numbers
appropriately instead of depending on knowing (or guessing)
the shift distance for the device major number.