net/rp-pppoe: update and clean up REINPLACE_CMD

Make sure that ${PREFIX} in some form is spliced before *_all_*
occurrences of /etc/ppp, in documentation, samples, etc.,
except in the CHANGES doc. file.

While here, heed the DOCS option (and do not install docs if unset).

Drop KERNEL-MODE-PPPOE documentation file, Linux-specific.

(Bump PORTREVISION, installed files change.)

Reported by: swills@ (by way of his REINPLACE_CMD cleanup endeavor)