Revert "pymapistore: remove sam_ctx from mapistore_connection_info - not used anymore"

This reverts commit 9695afb966e292d49add51c750dbb77832188417.
Revert "Merge pull request #55 from openchange/jkerihuel/_gnu_source_fix"

This reverts commit c9cb94df8f22d6d106e1a4e62e2bc69393348e22, reversing
changes made to 4c958f422bff35f20a8652b13bbd003918af3b3b.

- Include sam_ctx again in mapistore_connection_info - used by SOGo
pymapistore: return number of backends in mapistore_backend_list_backend_names