iutil.c: dump_solvables_queue.

Useful when debugging installonlies.
Limit the number of installed installonlies. (RhBug:880524)

apichange: hy_sack_set_installonly now takes a new parameter: the limit on the
number of present installonlies.

The implementation emulates exactly what yum does: look at all the *provides* of
the designated installonly strings, but limit the number of packages *per the
package name* basis.
goal: when sorting the installonly candidates, consider the running kernel.

Sack gets a hook to override testing the running kernel, useful in unit tests.