Patch to allow run time configuration of proofserv environment. The client
can pass a set of shell variables to PROOF. These variables can be used
to startup different versions of the proofserv, or they can instruct the
system to run proofserv under valgrind or tcheck, etc. Main changes:
- default wrapper script for proofserv
- modifies the build system for the wrapper script
- adds a list of environment variables to TVirtualProof, which are
  passed to the wrapper by proofd/xrootd.
- adds code to TProofServ to pickup these variables and pass them
  along to the slaves/submasters


git-svn-id: http://root.cern.ch/svn/root/trunk@16839 27541ba8-7e3a-0410-8455-c3a389f83636