Be user-friendly when opening a pfx/p12 file

Before this change the user had to provide a password callback, even if the
PKCS#12 encoded file has no or an empty password.

This changes the behavior, so we first try to open the file without a
password and only then ask the user to provide one.

This fixes #204.

Signed-off-by: Steffen Jaeckel <jaeckel-floss@eyet-services.de>

Makefile.am

@@ -116,6 +116,8 @@ EXTRA_DIST = \
 	src/tls_schannel.c \
 	tests/cert.pem \
 	tests/cert.pfx \
+	tests/cert.emptypass.pfx \
+	tests/cert.nopass.pfx \
 	tests/key.pem \
 	tests/key_encrypted.pem \
 	tests/res_query_dump.c