telnetd: Don't add empty lines to history

Also, does _accept_line() later.
telnetd: Fix multiline commands

I thought I had this right at some point in the past, but it wasn't working
today.  I think maybe this is correct again.
telnetd: Redirect stderr and stdin too

Previously, the Python telnet personality only handled stdout, so errors
got lost and input would appear to hang (while it tried to read from the
controlling terminal).  Now we do all three.
telnetd: Limit time spent executing commands

This attempts to keep runaway commands entered at the console from hanging
the cooperative thread.  If execution runs longer than two seconds (by
default), it's interrupted.  At least with CPython.