prefer Essentials over Removals in ordering score

Doing Removes early is good to have them out of the way, so they
don't break 'Inst' or 'Conf' chains, but scoring them above Essentials
means that we end up upgrading (many) less important packages before
we handle big stuff like libc6 or debconf which not only fails if those
less important packages have unannounced (strict) dependencies, but also
leads to having these packages unconfigured for a long time triggering
bugs in maintainer scripts for no good reason (#708831).

So this commits sets the default value for remove scores to 100, which
is below the one for essentials (200) and a lot lower than the previous
default value (500).
fix priority sorting by prefering higher in MarkInstall

Used to work until a certain (here unnamed) person came along and used
the wrong operator causing low-priority packages to be sorted above
high-priority packages while choosing a provider in commit
 2b5c35c7bb915dbd46fefd7c79f05364ba22f93b from Nov 2011
try all providers in order if uninstallable in MarkInstall