Extend process-lines to allow exit status handling

Extend process-lines to allow exit status handling

* subr.el (process-lines-handling-status): Extension of the old
process-lines, with more flexible handling of the exit status.
(process-lines): Old API implemented using the new function.
(process-lines-ignore-status): Another use of the new function -
return the output lines regardless of the exit status (bug#1321).