Fix BIO_get_fp compatibility

[olszomal]

Avoid undefined behavior with BIO_get_fp() by replacing BIO_new_file() with fopen() + BIO_new_fp()

[mtrojnar]

Thank you. Our osslsigncode seems a bit inconsistent about using BIO_new_file() vs fopen()/BIO_new_fp(). I think it would be useful to only use fopen()/BIO_new_fp(), preferably wrapping them in a new function in helpers.c. This way, we would never need to guess whether it's safe to use BIO_get_fp() for a BIO or not. What do you think?