GnuTLS challenge signing

GnuTLS challenge signing

This is the GnuTLS implementation for signing the challenge. Variables
added to store the paths for the second set of credentials.

Challenge response comprises two elements: an identity and signature.
The identity is the user certificate, represented as a PEM-encoded PKCS7
certificate chain. The challenge is signed with the user certificate
and submitted back to the server as a PEM-encoded signature.

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