zero pad DHE public key in ServerKeyExchange message for interop

Some versions of the Microsoft TLS stack have problems when the DHE public key
is encoded with fewer bytes than the DHE prime.

There's some public acknowledgement of the bug at these links:

https://connect.microsoft.com/IE/feedback/details/1253526/tls-serverkeyexchange-with-1024-dhe-may-encode-dh-y-as-127-bytes-breaking-internet-explorer-11
https://connect.microsoft.com/IE/feedback/details/1104905/wininet-calculation-of-mac-in-tls-handshake-intermittently-fails-for-dhe-rsa-key-exchange



This encoding issue also causes the same errors with 2048-bit DHE, if the
public key is encoded in fewer than 256 bytes and includes the TLS stack on
Windows Phone 8.x.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1320)