Fixes #824

Exporting variables in a dynamic library is a PITA. Let's keep
these values internally and wrap them through a helper method.

This doesn't break the external API. @arrbee, aren't you glad I turned
the `GIT_ATTR_` macros into function macros? :sparkles:

git_config_find_* does not set a git error

odb: allow creating an ODB backend from a packfile index

SSL isn't the only way that a transport can be encrypted. The new name
will make it easier to merge the SSH support.

Using the callbacks makes it clearer and reduces the amount of #ifdefs
and ifs and we need in the code.

Instad of each transport having its own function and logic to get to
its refs, store them directly in transport.

Leverage the new gitno_buffer to make the parsing and storing of the
refs use common code and get rid of the git_protocol struct.

This allows us to add capabilitites to both at the same time, keeps
them in sync and removes a lot of code.

gitno_buffer now uses a callback to fill its buffer, allowing us to
use the same interface for git and http (which uses callbacks).