* Update TLS protocol support
TLS1.0 is past PCI's EOL; BEAST is no more
Enable elliptic curve key exchanges
Do not enable the smallest ECs for security
Do not enable the largest ECs for computational time
Do not enable 25519 (sad) because it doesn't go across the wire
Drop non-PFS key exchanges
Drop ARC4, Blowfish, DES, genprime, XTEA code
Drop renegotiation support completely
It takes so much heap that it's not likely to work out well
Tidy handling of SSL_BUFFER_SIZE
Update docs
Drop mention of startcom, since they are no more, for letsencrypt
* Update mbedtls to 2.7.7
Preserve our vsnprintf and platform hacks
* Introduce TLS maximum fragment size knob
Reduce buffer size to 4Ki by default and advertize that. That's the
largest we can advertize with the TLS MFL extension, so there's no
point in making them larger. The truly adventurous can re-raise
SSL_BUFFER_SIZE and undefine the SSL_MAX_FRAGMENT_LENGTH_CODE and get
back to the earlier behavior.
* Default to mbedTLS debug with DEVELOP_VERSION
6e95d74fbd