FIXES: #1252

This adds to the *machineid* module (which generates random UUIDs
for DBus and systemd) another key to configure generation of
a urandom pool in the target from the entropy in the host system.

FIXES #1253

 - Improve documentation of the settings
 - If sysconfigSetup is true, **only** setup sysconfig and ignore
   the rest. This seems to be consistent with existing openSUSE-
   derivative distro's, which set displaymanagers to something
   nonsensical.

 - the *mount* module inserts a rootMountPoint without trailing /
   into global storage, so we can't assume that here. On the other
   hand, the paths passed in to the Worker functions are absolute
   paths -- adjust the tests to follow that.

 - The code in Workers.cpp assumes that rootMountPoint ends in a /
   so that it can have filenames appended easily; make the tests
   fit that assumption, but still need to check that it is so in
   production.

 - refactor running the command into a helper function,
   to deal with the regular if-command-failed-then-complain pattern.
 - mark parameters as unused.
 - move distinction about kind of DBus file up into the MachineIdJob
   and remove the enum that marked it.

 - Testing some of the functionality that's been added just now:
   - copyfile fails, buggy implementation
   - poolsize fails, buggy implementation
   - removefile not tested

 - read-urandom or copy-existing-file are implemented
 - fairly chatty on failure
 - needs tests (probably the implementation should be moved to
   a separate file and unit-tested)

 - keep the rootMountPoint and the path-with-random-data separate
   instead of concatenating them at the beginning. Then we can
   use the "clean" names also within the host system.

 - this could be named isValid() instead, but basically the idea
   is that this code makes sense:

      JobResult r = do_thing();
      if ( !r ) { /* Error happened! */ return r; }
      /* Carry on .. */

 - remove existing files for each kind of random-generation
   that is enabled. There's a helper function for the case that
   Cala is no longer setuid and needs help to remove those files
   from the target (e.g. a setuid helper).

 - document module's purpose
 - document existing configurations
 - deprecate "symlink" and introduce more-accurate "dbus-symlink"
 - add new configurations for upcoming entropy file

FIXES #1181
FIXES #1188

You can now copy single files from the source image to the target.
You can now copy directories from the source image to the target.

 - move all the examples to an EXAMPLES section
 - leave one single-file example enabled in the configuration

 - Just use the existing rsync code, which can do both
   files and directory trees.
 - The existing code assumed we were always copying directories.
   Now double-check beforehand.