Log Message:
Add a new "doc" option to install multimedia/ffmpeg[23] HTML documentation,
disabled by default.

Fix PR pkg/51113 reported by Joern Clausen.
ffmpeg needs texi2html features that can be not available in the version
provided natively by the platform.

While here also add a PRINT_PLIST_AWK to automa{g,t}ically print a PLIST that
need less manual intervention.

Bump PKGREVISION.