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

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.