README was renamed
rebuild-cache: Add a new subcommand

This adds the subcommand to the argument parser, and prepares the code
skeleton for the actual impleemntation.
rebuild-cache: Clean the Yum cache
yumbase: Access the config early

Under the hood, accessing the config calls the base._getConfig() method,
which itselfs does a bunch of preparation stuff, like reading the
configs, but also... setting up the logging.

That seems like a good idea to do early on, especially as we'll soon
(i.e next commit) need to do some logging very early on.
rebuild-cache: Prevent usage of local cache

It makes no sense to force usage of the local cache if we are rebuilding
it from the remote repositories.
rebuild-cache: Don't prepare the Yum base twice
logging: Ignore the presto logs

The presto plugin logs some underied output when cleaning the local
cache (it cleans the deltarpms it had downloaded).

We don't use presto at all at the moment (we don't generate deltarpms)
so there's no harm in completely ignoring all of presto output for now.
yumbase: Make filtering code slightly more sane

I'm reworking this part, as it's pretty ugly.

Further commits will follow.
yumbase: More filtering sanity

More is coming...
yumbase: Filters and patterns, reworked

We were using that horrendous __filter_and_patterns_preprocessor()
function in 3 different places: install, remove and list.

But looking better, all 3 were interested in the stuff it does to
patterns, but only one (list) cares about this filter stuff.

This commit scratches all that:
- the ugly function is now simpler, as it only deals with patterns, not
  filters
- list() cares about filters, so it handles them itself
list: Allow hiding some security modules

The distro vendor can now produce security modules with a group of
'nbhidden', and they won't be listed **by default** by nbyum.

However, this only affects `list`, all other operations work just fine
for these "hidden" security modules.

Finally, we introduce a new --show-hidden option.
list: Document the hiding of some packages
The command can only run as root

Since commit 515108a7d74de97eca86e7a5edfb783eba01ec7d, the run()
function itself must be run as root (to obtain the Yum lock).

As a result, it doesn't make much sense any more to ensure the user has
the right privileges for individual functions.
utils: Drop the ensure_privileges decorator

As mentioned in the previous commit, nbyum pretty much requires being
run as root for all operations now.

As such, there's no point in complicating our lives with a decorator,
let's just test the uid at startup.
check-update: Rework the 'last update' stuff

I'm about to introduce a new 'last-updated' command, this is preparatory
work for that.
utils: Rename get_local_datetime

I'm about to introduce a similar function, which will return the sam
date from the same timestamp, but in a different format.
utils: Convert a timestamp to the ISO datetime format
last-updated: New command
last-updated: Don't process the local metadata

That's really unneeded...
last-updated: Document the new command