AuthorizedKeysCommand: add an option for alternate owner (bug#2276)

Currently the owner of AuthorizedKeysCommand must be root.

A setup in which sshd is running as non root, can enjoy a complete
and secure environment even if the AuthorizedKeysCommand is owned by a
different user.

This patch adds AuthorizedKeysCommandOwner option to control the
ownership check of the AuthorizedKeysCommand. Default is root, so no
change is done without explicit request.