Move the TLS padding extension under an SSL_OP_TLSEXT_PADDING option, which
is off by default (instead of being enabled unconditionally).

The TLS padding extension was added as a workaround for a bug in F5 SSL
terminators, however appears to trigger bugs in IronPort SMTP appliances.
Now the SSL client gets to choose which of these devices it wants to
trigger bugs in...

Ported from OpenSSL.

Discussed with many.

ok miod@
Document SSL_OP_TLSEXT_PADDING.

From OpenSSL.