Various SASL fixes.

Export $tls_bits new expansion variable (not yet documented).
Fix tls-gnu.c so that ciphername string construction uses bit-count, not byte-count.
Avoid hard-coding primary_hostname in first call to init Cyrus SASL.
Cast fix for function pointer (Cyrus-SASL uses void params in struct entry funcptr, so need to cast).
Many more debug statements in cyrus_sasl.c
Pass external SSF from TLS cipher into Cyrus SASL initialisation.
Detect when we can't get an identity from SASL properties (error out correctly).
Detect when SASL negotiated a protection layer and error out, since we do not support those.