Make crypto backend configurable.

With this commit, no actively used code outside sigver.py really
knows about xmlsec1.
Remove unused module.

Roland believes this module can be removed - leftovers from some
earlier refactoring.
Avoid hard xmlsec_binary requirement for ECP client.

By providing an already initialized Config() instance, an ECP
client using another CryptoBackend than xmlsec1 can be instantiated.
Remove obsolete xmlsec_path from pathutils.