the need to also create the entire ports tree during a `make release'.
The main motivation behind this is that handling the ports tree still
takes a huge amount of time due to the large number of directories
involved, even on modern disks.
The solution is to establish a list of dependent ports that are
minimally required. This list needs to be manually maintained in case
the doc ports toolchain changes, and has thus been broken out into a
separate file Makefile.inc.docports. (release/Makefile has gotten
overly lengthy already anyway.)
Discussed with: bmah, nik
Reviewed by: bmah
