Factor out keycert loading

Factor out keycert loading

The objective is to have load_keycert fetch credentials for both
establishing the TLS session and responding to the multicert auth
challenge. The main changes are: load_keycert that is parameterized by
key_path, cert_path, and password; have secondary functions
parameterized by password; load_certificate stub to call load_keycert
and match caller impedance, including maintaining
gnutls_certificate_credentials and gnutls_session dependencies.

Signed-off-by: Tom Carroll <incentivedesign@gmail.com>