read_acl_{comments,seq}: rename "line" to "lineno"

The word "line" is used to refer to the content of a line, and a few
places are already using "lineno".  Change all to "lineno" so that we
can use "line" consistently.
read_acl_{comments,seq}: switch to next_line

Rather than use a fixed length buffer, use next_line.  This let's us
handle any arbitrary length and avoid the non-portable PATH_MAX.

Fixes bug 27388 in the acl tracker.