Repurposed the /p4/common/site directory.
This directory will exist on a a fresh SDP install, but will
be empty save for a ReadMe.txt file explaining that it is
to be used to make local extensions to the SDP, and that
anything in here is not supported. The coming automated
SDP upgrade procedure will know to ignore /p4/common/site
The p4_vars ensures that /p4/common/site/bin is in the PATH.