As part of this, removing so much warning spew let me fix some minor
real issues in debug logging.
+PP/32 GnuTLS was always using default tls_require_ciphers, due to a missing
+ assignment on my part. Fixed.
+
+PP/33 Added tls_dh_max_bits option, defaulting to current hard-coded limit
+ of NSS, for GnuTLS/NSS interop. Problem root cause diagnosis by
+ Janne Snabb (who went above and beyond: thank you).
+
+PP/34 Validate tls_require_ciphers on startup, since debugging an invalid
+ string otherwise requires a connection and a bunch more work and it's
+ relatively easy to get wrong.
+
Exim version 4.77
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