Stub out some more of PKCS7.

This is to help with cryptography.io compatibility. We don't implement
any of the flags (PKCS7_sign checks flags == PKCS7_DETACHED), but
cryptography.io now depends on the constant and PKCS7_SIGNER_INFO type
being available.

(cryptography.io also wants some new functions, but I think it's easier
to stub those out externally for now. If we need to actually enable
those features, we can look at actually implementing more of
PKCS7_sign.)

Change-Id: Id8419e34a68c04d4894417c7d6b13c1952d0bb88
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/46084
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>

include/openssl/pkcs7.h

diff --git a/include/openssl/pkcs7.h b/include/openssl/pkcs7.h
index cb6155f..d2d642a 100644
--- a/include/openssl/pkcs7.h
+++ b/include/openssl/pkcs7.h
@@ -101,6 +101,7 @@
 typedef void PKCS7_ENVELOPE;
 typedef void PKCS7_DIGEST;
 typedef void PKCS7_ENCRYPT;
+typedef void PKCS7_SIGNER_INFO;
 
 typedef struct {
   uint8_t *ber_bytes;
@@ -183,6 +184,7 @@
 #define PKCS7_NOATTR 0x100
 #define PKCS7_NOSMIMECAP 0x200
 #define PKCS7_STREAM 0x1000
+#define PKCS7_PARTIAL 0x4000
 
 // PKCS7_sign assembles |certs| into a PKCS#7 signed data ContentInfo with
 // external data and no signatures. It returns a newly-allocated |PKCS7| on