Make bntest be (mostly) file-based.

Test suite used from boring, written by David Benjamin.
Test driver converted from C++ to C.
Added a Perl program to check the testsuite file.
Extensive review feedback incorporated (thanks folks).

Reviewed-by: Emilia Käsper <emilia@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>