key share: added flags to gnutls_init() to modify its default behavior

key share: added flags to gnutls_init() to modify its default behavior

That way the application can adjust the range of keys generated
during client hello attempting to guess the server's algorithm.

Applications are intentionally not given the option to select the
algorithm in the key share, but rather chose from the prioritized
list of groups, to avoid a disconnect between the prioritized
groups, and the key share sent.

Relates #284

Signed-off-by: Nikos Mavrogiannopoulos <nmav@redhat.com>