stunnel: Add TLSv1.1 and TLSv1.2 support

The system copy of OpenSSL is too old to support TLS versions 1.1 and
1.2.  This matters because TLS v1.2 includes a number of ciphers that
are stronger than anything available in earlier versions, so people
who need to use those ciphers need TLS 1.2 support.

stunnel enables TLSv1.2 and the stronger ciphers automatically when
compiled with a sufficiently recent version of OpenSSL, so the change
here is just to add the usual "--with-brewed-openssl" option to the
stunnel formula.