Forgot to reset per-name penalty when it got locked by apply_decision.
This also fine tunes compare_package_preference() to always prefer
packages specified on command line speeding up calculation certain
complicated solutions.

Previously we would cache the penalty when evaluating the final
solution, and adding that until we backtrack to first topology
position changing that penalty. However, we can just keep track
of minimum penalty based on name state, and add it. This allows
us to bail out early on bad branches because we know in advance
how things will turn out.

Improves /etc/apk/repositories format so you can say:
http://nl.alpinelinux.org/alpine/v2.3/main
@edge http://nl.alpinelinux.org/alpine/edge/main
@testing http://nl.alpinelinux.org/alpine/edge/testing

After which you can pin dependencies to these tags using:
 apk add stableapp newapp@edge bleedingapp@testing

Apk will now by default only use the untagged repositories,
but adding a tag to specific dependency:
 1. will prefer that tag for the name
 2. allowing pulling in dependencies from that tag (though,
    it prefers untagged packages to satisfy deps if possible)

fixes #575

One can now say in dependency "!foo<2" which means, that if foo is
installed, it needs to be >=2, but it's not a required dependency.

Should now choose packages better if the best available version
is uninstallable for some reason.

Allow to select packages that conflict in case we are looking for
errors. This allows 'add --force' to install (on boot) the set of
packages with minimum conflicts.

 * each package name has two sorting positions, one which causes
   install_if triggers to be run, and other for bulk dependencies
 * fix also inverted ordering of package installations

 * the solver no longer does look-ahead locking of names
   (could be possibly optimized later); instead names are now
   always ordered strictly to properly detect the package names
   which are unsolveable
 * basic error tests added, so we can see the most likely problem
   in dependencies easily

Packages that need (re-)installation but which are not available,
are excluded now properly.

The first found solution is the most preferred one then.