sandbox: Change how content directory is specified

Firstly, allow specifying multiple --content options, and allow
specifying --content-non-recursive if you really want as well.

Also, don't generate an XDG user directories config. It seems a bit
weird to be doing this and setting each directory to the same location.
sandbox: Fix to use new-style config files

The config file format changed in commit
2b53cd5d3aaf046c413fd4afb188c16d7e611f48.
miner-fs: Give clearer warning when XDG user dirs are unconfigured

If Tracker can't resolve special dirs like &DOCUMENTS to real path
names, it now gives a warning like this:

    (tracker-miner-fs:7207): Tracker-WARNING **: Unable to get XDG user
    directory path for special directory &DOCUMENTS. Ignoring this
    location.

Previously (since commit 5f06c134f4f6da02027c813322e13c220b51cd0a) the
user would see this rather more scary output:

    (tracker-miner-fs:6046): GLib-GIO-CRITICAL **: g_file_new_for_path:
    assertion 'path != NULL' failed

    (tracker-miner-fs:6046): GLib-GIO-CRITICAL **: g_file_equal: assertion
    'G_IS_FILE (file1)' failed

    (tracker-miner-fs:6046): GLib-GObject-CRITICAL **: g_object_unref:
    assertion 'G_IS_OBJECT (object)' failed

This situation is rare (you need to manually change or break
XDG_CONFIG_HOME to really see it) but it's always nice to avoid showing
critical errors!