apt's dpkgpm.cc WriteApportReport function should gather more data

While fixing bug 1813464 it occurred to me that it is not obvious that the dist-upgrader tarball includes its own copy of invoke-rc.d and that this has been complicated to debug more than once. The apport information in that report was gathered by apt's dpkgpm.cc code and I think we should extend that to gather additional information from the environment at the time that the package installation failed. For example the dist-upgrader sets the following:

        os.environ["PATH"] = "%s:%s" % (os.getcwd()+"/imported",
                                        os.environ["PATH"])

Gathering path from the environment would have made diagnosing bug 1813464 a lot easier. I tested modifying dpkgpm.cc with the following:

   if(const char* env_p = std::getenv("PATH"))
      fprintf(report, "EnvPATH: %s\n", env_p);

I then installed the crash-in-postinst package from the daisy-plucker-seeds PPA and received a crash report with the EnvPATH key and value. Its worth noting that when apport gathers environment information only a white listed set of keys is gathered to prevent disclosing potentially sensitive information. So we should look at what environment variables the dist-upgrader sets and gather those at a minimum.