libjpeg-turbo installs its own unique library (libturbojpeg.so) and
a drop-in replacement for libjpeg.so.

This commit splits them apart into two ports:
graphics/libjpeg-turbo which installs only libturbojpeg.so
graphics/jpeg-turbo which installs the libjpeg.so replacement

This will allow ports to depend on libturbojpeg.so without causing
pkg conflicts.

Approved by: maintainer