wr_mon: don't use nonblocking input

For unknown reasons, setting stdin to nonblocking mode has global
effects, and thus every later command will get EAGAIN instead
of a blocking read:

   vi: can't read user input

   wrs-192.168.16.242#cat
   cat: read error: Resource temporarily unavailable

But wr_mon doesn't need non-blocking mode, because it uses poll on stdin.
So instead of fixing, remove the source of the bug.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>