Automatically use OpenSSL by default on Linux and FreeBSD if available

Since https is almost ubiquitous nowadays we should support it by
default whenever possible.  When building our own curl, we already
automatically enable SSL/TLS support on Windows and OS X by using the
OS-native APIs.  On UNIX platforms we need to use OpenSSL but have not
done so by default before, leading to possible user confusion when https
transfers fail later.  Fix this by searching for OpenSSL quietly and
enabling use of it automatically if it is found.

Do this only on Linux and FreeBSD for now because on other UNIX
platforms (e.g. AIX, HP-UX, SunOS) it seems too easy to find an
OpenSSL that is not compatible with the target compiler.