Review: Inital pass at searchpath impl

Malcolm Humphreys <malcolmh...@...>

Inital pass at searchpath impl
- added environment variable string expansion code in PathUtils.h (GetEnvMap() & EnvExpand())
- added FileExists() function to PathUtils.h
- added searchpath functionality into Config:: class (setSearchPath(path), getSearchPath(expand), findFile(filename)) this hasn't be plugged into anything yet.
- added so we can pass in some env vars for the unit tests


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