Feature an optional crypto-specific macro to rsa sign a data fragment vector

OS/400 crypto library is unable to sign a precomputed SHA1 hash: however
it does support a procedure that hashes data fragments and rsa signs.
If defined, the new macro _libssh2_rsa_sha1_signv() implements this function
and disables use of _libssh2_rsa_sha1_sign().

The function described above requires that the struct iovec unused slacks are
cleared: for this reason, macro libssh2_prepare_iovec() has been introduced.
It should be defined as empty for crypto backends that are not sensitive
to struct iovec unused slack values.

src/openssl.h

@@ -114,6 +114,8 @@
 
 #define _libssh2_random(buf, len) RAND_bytes ((buf), (len))
 
+#define libssh2_prepare_iovec(vec, len)  /* Empty. */
+
 #if OPENSSL_VERSION_NUMBER >= 0x10100000L
 #define libssh2_sha1_ctx EVP_MD_CTX *
 #else