cred: tighten username rules

The ssh-specific credentials allow the username to be missing. The idea
being that the ssh transport will then use the username provided in the
url, if it's available. There are two main issues with this.

The credential callback already knows what username was provided by the
url and needs to figure out whether it wants to ask the user for it or
it can reuse it, so passing NULL as the username means the credential
callback is suspicious.

The username provided in the url is not in fact used by the
transport. The only time it even considers it is for the user/pass
credential, which asserts the existence of a username in its
constructor. For the ssh-specific ones, it passes in the username stored
in the credential, which is NULL. The libssh2 macro we use runs strlen()
against this value (which is no different from what we would be doing
ourselves), so we then crash.

As the documentation doesn't suggest to leave out the username, assert
the need for a username in the code, which removes this buggy behavior
and removes implicit state.

git_cred_has_username() becomes a blacklist of credential types that do
not have a username. The only one at the moment is the 'default' one,
which is meant to call up some Microsoft magic.
Merge pull request #2274 from libgit2/cmn/ssh-expect-username

cred: tighten username rules