Fix client's command abort at beginning of first input line

Commit 3cceb59b (v4.3.26) fixed the client to abort commands reliably
on ^C, even when it arrives at the beginning of an input line.  Except
it didn't work at the beginning of the first input line, because
input_eol was initialized to zero.

Easily fixed, but "end of line" isn't quite right there.  Revert sense
and rename to partial_line_sent.