only way to use PKCS12 files with password is specify it on command-line
"--cert my.p12:secret" or set via CURLOPT_SSLCERTTYPE option.

this patch adds invocation of PEM_def_callback() to grab password
interactively if password not specified,
like SSL_CTX_use_certificate_chain_file() does for PEM files.

also patch allow to use empty passwords on PKCS12 files.

code based on load_pkcs12() function from openssl/apps/apps.c

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