mknod(2): Restrict functionality to creating FIFOs.

mknod(2): Restrict functionality to creating FIFOs.

Now that we have devfs(5) for handling our device nodes, we can retire
part of mknod(2) functionality and restrict it to what POSIX requires:

"The only portable use of mknod() is to create a FIFO-special file.
 If mode is not S_IFIFO or dev is not 0, the behavior of mknod() is
 unspecified."

In-discussion-with: beket