Add explicit return codes to frontend functions

On successful invocation, all frontend functions should return 0.
Check for SSL_CONFIG file and abort when missing
Define OPENSSL_CONF to match SSL_CONFIG during vars processing

This prevents OpenSSL from complaining if its default expected
openssl.cnf file is missing; in particular, this hides an unnecessary
warning on Windows every time openssl is called.
Support shell.w32 environment when searching for vars

The $0 variable within shell.w32 uses DOS-style backslash directory
separators, so formatting the 'vars' path relative to the program
location requires special handling.
Update testing-setup.sh dev script for new vars format
Update TODO