gnu: Populate /etc upon startup.

* gnu/packages/patches/dmd-getpw.patch: New file.
* gnu-system.am (dist_patch_DATA): Add it.
* gnu/packages/system.scm (dmd): Use it.
* gnu/system/vm.scm (system-qemu-image): Don't explicitly populate /etc;
  don't add ETC to the GC roots.
* gnu/system/dmd.scm (dmd-configuration-file): Add 'etc' parameter.
  [config]: Populate /etc from there.