RFE: -p without an argument should prompt for a password

[kevinclevenger]

With password re-entry confirmation similar to 'openssl enc ...'

[badiku]

I add this, prompt for password when no -p and no -P:

    if (ctx->has_key == 0) {
        if (ctx->encrypt == -1) usage();
        else {
        	printf("password:");
        	char password[512];
        	scanf("%s", password);
        	derive_key(ctx, password, strlen(password));
        }
    }