Add a notion of program search path to the ProgramDb

Previously we would just use the normal system $PATH for finding
programs (unless the Program provided some custom method). Now the
ProgramDb has its own notion of the search path and we use that for
finding programs (by default). The search path can be either specific
directories or the system search method (ie $PATH on unix and something
similar on Win32). The default search path is just the system one.

In addition, this search path is passed on to programs when we invoke
them as the $PATH env var.