Add an initial regress test for pkcs7.

This currently fails when signing/verifying with a detached signature.
remove use of internally-deprecated OPENSSL_malloc/free
disable warnings about constant addresses in gcc 4.6 and later
do not include <openssl/modes.h>, modes_lcl.h already does
add string.h for memset
fix format string should not be string literal error from clang

pkcs7test.c:105:8: error: format string is not a string literal
(potentially insecure) [-Werror,-Wformat-security]
              warnx(msg);