context: Respect install root for rpmdb Packages monitor

If we're using a different root, don't monitor the system packgaes
database.
context: Error if we can't find any package matching provided name

In my opinion, yum just emitting a warning when it can't find a
provided package name is a very irritating flaw.  For example, when I
copy/paste a list of build dependencies and it can't find one,
proceeds to install the others, then I wonder why my software isn't
building.

Let's not reproduce that behavior.
Add some g_debug()

Trying to debug libhif.
source: Add wall clock comment to docstring, note behavior with MAXUINT

When talking about time, there can be monotonic or real, let's clarify
which is expected.
source: Add name of failing repository

Easier to debug.
context: Add accessor in -private for HifState

Continuing my thread of not using HifContext other than as a
convenient property bag, add an accessor for HifState so that I can
drive things directly with it.
source: Only set librepo option if value is set

librepo right now has some magical (and deprecated) handling of the
LRO_MIRRORLIST option; for one thing, it calls strstr() on the url and
if it contains "metalink", it also sets it as a LRO_METALINKURL.

(Note the default Fedora repo files have mirrorlist=.../metalink/...)

But libhif circumvents this by explicitly setting NULL on the handle
even if the yum repository file didn't have it.  Change the setters to
skip setting the options to NULL, leaving them as default.

This fixes rpm-ostree composes.
context: Create an initial sack

I'm not using the high level context APIs, for one thing because they
want to load the repository state directly, whereas I want more
control over the enabled repositories, for example.

So this patch just sets up a default sack that I can use when running
hif_sack_add_source() directly.