gen_req() no longer alters the global EXTRA_EXTS variable

When forming the OpenSSL config replacement text, gen_req() should not
alter the global variable as it is referred to again by sign_req() when
the wrapper-commands build-*-full are called.

This change localizes the replacement text to avoid competing over the
global variable.

Signed-off-by: Josh Cepek <josh.cepek@usa.net>
Based-On: patch submitted by Joe Hu <i@sapika.ch>
Fixes: Issue #25