implement updateinfo caching

Things are a bit tricky, as the updateinfo is not an extension
for existing solvables, but contains its own set of solvables.
libsolv should offer more support for this kind of setup.
doc: fixed typo
added get_pkgname_prefix function for parsing name from nevra global pattern
added nevra_match filter flag to query C API
added python bindings to nevra_match attribute filter
fixed clang warning of uninitialized variable