Support SDP hosts without p4d, p4p/p4broker or p4dtg only.
The SDP mkdirs.cfg describes SERVER_TYPE values of p4broker
and p4proxy, but they need to fully implemented.
Some things to address:
* The default shell environment mechansim (sourcing p4_vars)
gives ugly errors if P4ROOT doesn't exist -- but P4ROOT wouldn't
be expected to exist on a host running only p4broker or p4p.
* mkdirs.sh can't resist putting p4d in /p4/N/bin when setting
up a new instance, but shouldn't based on the SERVER_TYPE.
* upgrade.sh might have a cow if there is no P4ROOT.
host with only p4p and/or p4broker, it should do the right
thing and upgrade them. Something like: service stop, replace
exes, do symlink adjustment, and service start. (See SDP-278;
this may already be done. Double check).