string: Add discard feature for for_each_line().

string: Add discard feature for for_each_line().

This adds the new FELF_DISCARD_FIRST flag which instructs
for_each_line() to discard everything up to the next newline.

The new feature is only used by para_gui. When para_gui detects
a partial overlong input line, it ignores this data and passes the
FELF_DISCARD_FIRST flag in subsequent calls to for_each_line(). Hence
output continues at the next line.