Fix #45: Handle relative paths in `run-program`

This is basically the solution proposed by Elias Pipping with a few
minor tweaks.

    - In `run-program`, don't merge `program` with the "path:"
      search-list.  `spawn` will handle this.
    - In `spawn`, if the first call to execve fails, instead of trying
      "/bin/sh", use "/usr/bin/env" which will use the user's PATH if
      necessary to find the program.