Raise the default for sendmail client connections to 1024-bit DH
parameters to imporve TLS/DH interoperability with newer SSL/TLS
suite, notably OpenSSL after FreeBSD 10.1-RELEASE-p12 (FreeBSD-
SA-15:10.openssl).

This is MFC of r284436 (gshapiro), the original commit message
was:

The import of openssl to address the FreeBSD-SA-15:10.openssl security
advisory includes a change which rejects handshakes with DH parameters
below 768 bits. sendmail releases prior to 8.15.2 (not yet released),
defaulted to a 512 bit DH parameter setting for client connections.
This commit chages that default to 1024 bits. sendmail 8.15.2, when

released well use a default of 2048 bits.

Reported by: Frank Seltzer
Errata Notice: FreeBSD-EN-15:08.sendmail
Approved by: so