Enable support for modern SSL (#12524).

 * --ssl-protocol= option now accepts 'tlsv1.2', 'tlsv1.1', 'tlsv1.0'
   and 'default' as well as the existing 'tlsv1', 'sslv3', and 'any'.

 * The default is now none of the above, but rather QSsl::SecureProtocols,
   which means "whatever subset of ANY is still considered secure and also
   supported by the OpenSSL library in use".  (As of this writing, Qt's idea
   of "still considered secure" includes everything from SSLv3 on up, which
   is technically wrong -- SSLv3 has known breaks -- but we can live with.
   Qt currently doesn't have a way to select "TLSv1.0 and up".)
Make ciphersuite selection configurable; match Chromium by default. (#12524)

New option --ssl-ciphers takes a colon-separated list of OpenSSL
cipher names and sets the client cipher list to exactly that list.

The default for this option is arranged to match Chromium 35, which
has had its cipher selection optimized for the modern Web
(see https://briansmith.org/browser-ciphersuites-01.html for
rationales).  (Newer versions are the same except that they also add
ChaCha20-based ciphersuites, which OpenSSL 1.0.1 hasn't yet picked up.)