auth: fail pam and internal authentication attempts if no username is provided (Issue #180)

When the Mercurial client communicates with a server over HTTP, it will always
first try to perform operations unauthenticated before providing credentials.
Authentication attempts without credentials is usually pointless and will just
slow operations down.

Some authentication plugins (such as LDAP) already skipped these
unauthenticated requests. Now, do the same for other authentication plugions.

Other authentication plugins also skip if no password is provided ... but that
doesn't seem necessary.