Fix checkresult output processing (again).

Similar to 0834195418570a457e525334b743ffa91e548dbc, this commit
unescapes plugin output submitted via external commands.

The unescape_check_result_file_output() (previously
unescape_newlines()) function that handles the unescaping has been
renamed and made externally visible so it can be used from base/utils.c
and base/commands.c.

The previous braggadocio should now be correct: All checkresult output,
single or multiple line, and from all sources, should now be parsed
correctly by Core.