Rename pem_password_cb to wc_pem_password_cb.

Recently, we had a wolfEngine customer report a compilation error because
wolfSSL and OpenSSL both define the typedef pem_password_cb. The solution is to
namespace our typedef with the wc_ prefix. In order to not break existing code
that relies on wolfSSL providing pem_password_cb, if OPENSSL_COEXIST is not
defined, we define pem_password_cb as a macro that maps to wc_pem_password_cb.