Remove the global "struct cm_cred_struct" and associated calls, make

Remove the global "struct cm_cred_struct" and associated calls, make
callers pass in a struct user_auth_info * instead. This commit causes
smbc_set_credentials() to print out a message telling callers to use
smbc_set_credentials_with_fallback() instead, as smbc_set_credentials()
has a broken API (no SMBCCTX * pointer). No more global variables used
in the connection manager API for client dfs calls.
Jeremy.