Fix first output bug introduced in last commit

The old 'got some data since last time' check seemed redundant with the new
output-once-per-second code, but was still necessary for the case where no
data had been received yet.

Now also check that we have some data.
Use real term width in the clreol code

Factor the TIOCGWINSZ code into a function shared by print_alternate_output
and the clreol code.