Filter HSTS response headers to allow cert override

Also remove HTTP Strict Transport Security (HSTS, RFC 6797) headers from
HTTP responses.  With HSTS active, the user is not allowed to accept
untrusted certificates.
Improve manual page re protocols and scalability

Issue:		#42
Make SSLv2 support opt-in instead of opt-out

Migrate knobs from DISABLE_SSLV2_CLIENT and DISABLE_SSLV2_SERVER to
WANT_SSLV2_CLIENT and WANT_SSLV2_SERVER and remove the
DISABLE_SSLV2_SESSION_CACHE knob entirely, automatically including the
respective code if SSLv2 support is compiled into SSLsplit.
SSLsplit 0.4.9 release