py: api: hawkey.Repo() does not accept cost keyword arg.

Related: RhBug:1048973
py: better error checking in repo-py.c:set_int().
priorities: change the meaning of the setting---lower number=better prio.
selector: added file filter (Related: RhBug:1100946)
Add advisorypkg object.
Add advisorypkglist object.
Add hy_advisory_get_packages method.
tests: add tests for advisorypkg object.
py: add _hawkey.AdvisoryPkg type.
py: add _hawkey.Advisory.packages attribute.
tests: py: add tests for _hawkey.AdvisoryPkg type.
api change: deprecate hy_advisory_get_filenames.
api change: drop deprecated hy_package_get_update_*.
api change: py: deprecate _hawkey.Advisory.filenames.
build: 0.4.18-1
doc: deprecation policy.
py: detailed error reporting.

Have a string field too as the simplest way to tell what failed. Currently
only used for HY_E_IO error.

Related: RhBug:1117789
Hide hy_err_str from errno.h

Nobody needs to see it..
hy_err_str: it's best to make it static.

Consulted with <pmachata@redhat.com>.
api change: py: convert Advisory, AdvisoryRef and AdvisoryPkg attributes to Unicode.