sanon: Fix flags and ctx export/import confusion
We were passing SANON flags to _gss_mg_import_rfc4121_context(), which
wants GSS flags. Meanwhile, I broke gss_inquire_context() on imported
SAnon contexts when I did my review of SAnon.
This commit fixes both issues and removes SANON_FLAG_*, which were only
ever needed because of a flag to track whether a context was locally
initiated or accepted. Now we use a separate int field of the sanon_ctx
to track whether a context was locally initiated. Once an SAnon context
is fully established, we rely on gss_inquire_context() on the rfc4121
sub-context for all metadata that isn't the initiator and acceptor names
nor the mechanism OID.