libssh2 used to explicitly check for libgcrypt and default to OpenSSL.
Now all possible crypto libraries are checked for explicitly, making
the addition of further crypto libraries both simpler and cleaner.
Make sure we don't clear or reset static structs after first init so
that they work fine even when used from multiple threads. Init the
structs in the global init.
Help and assistance by: John Engstrom
Fixes#229 (again)
Added crypto.h that is the unified header to include when using crypto
functionality. It should be the only header that needs to adapt to the
underlying crypto library in use. It provides the set of prototypes that
are library agnostic.
