refdb_backend: improve callback documentation

The credential structures are now opaque and defined in
`sys/credential.h`.  However, we should continue to provide them for
backward compatibility, unless `GIT_DEPRECATED_HARD` is set.

The callbacks are currently sparsely documented, making it really hard
to implement a new backend without taking a look at the existing
refdb_fs backend. Add documentation to make this task hopefully easier
to achieve.

Fixes #5428

This commit also switches our SOVERSION to be "$MAJOR.$MINOR" instead of
"$MINOR", only. This is in preparation of v1.0, where the previous
scheme would've stopped working in an obvious way.

cred: change enum to git_credential_t and GIT_CREDENTIAL_*

We avoid abbreviations where possible; rename git_cred to
git_credential.

In addition, we have standardized on a trailing `_t` for enum types,
instead of using "type" in the name.  So `git_credtype_t` has become
`git_credential_t` and its members have become `GIT_CREDENTIAL` instead
of `GIT_CREDTYPE`.

Finally, the source and header files have been renamed to `credential`
instead of `cred`.

Keep previous name and values as deprecated, and include the new header
files from the previous ones.

Stop returning a void for functions, future-proofing them to allow them
to fail.

Stop returning a void for functions, future-proofing them to allow them
to fail.