When it is set, include the NSS headers and libraries as
appropriate.  Doesn't actually use them yet, though.

Now the address-set code and the digest-set code share the same
backend.

Closes ticket 26510

I am calling the crypto library "crypt_ops", since I want
higher-level crypto things to be separated from lower-level ones.
This library will hold only the low-level ones, once we have it
refactored.