Add support for testing supported curves

since early versions of 1.0.2 openssl supports -curves command line
option, it allows us to set the curves advertised as supported

use the same approach to testing: advertise all, check what server
accepts, remove the accepted from list, repeat. When server aborts
connection or selects non ECC cipher, we know that we've tested all.
that's still not boolean - and I don't think bash has booleans at all

the alternative is to use numeric values and a as a result magic values or define variables that mean boolean true and boolean false

I think code that uses strings for true and false values is more readable