diffie-hellman: Handle dh_exponent_ansi_x9_42 as a boolean setting

While it was always documented as boolean setting, the option is currently
handled as integer value, for which yes/no values do not work. Instead the
default of TRUE is used for a no value.

The option has been moved a lot during the last years, and in some locations
was handled as bool, in some as integer. In the latest codebase it
congruently used integer, which is actually not what is documented and used
in testing.

Fixes #781.