Highlighting of locations in selected source buffers.
If a number, highlight the locus in `next-error' face for the given time
in seconds, or until the next command is executed.
If t, highlight the locus until the next command is executed, or until
some other locus replaces it.
If nil, don't highlight the locus in the source buffer.
If `fringe-arrow', indicate the locus by the fringe arrow
indefinitely until some other locus replaces it.