Limit use of small DH keys in Node

[mhdawson]

This is to close on the related issues mentioned in:

#25509

The summary over the overall mitigation is:

• update to openssl-1.0.1o, this will protect the tls client, unfortunately there will be no way to revert to previous behavior and will prevent users who have upgraded from connecting to servers using a smaller key than 768 (DONE in 0.10.39 and 0.12.5)
• To add protection on the server side, pull in tls: make server not use DHE in less than 1024bits for fix agaist Logjam Attack node#1739 but add an option/env variable to revert to previous behavior. This is secure by default but minimizes breakage in the LTS streams but providing a way out if necessary. From earlier comments seems like this is only needed for 0.12.X
• Leave out Add a new option to limit DH key size in tls connect node#1831 as we already have protection for the client from the openssl upgrade and it depends on some functionality not available in openssl-1.0.1. 1831 will be in place for the next major node stream.
• drop modp1 (option 1 in Remove (or discourage) small DH groups in crypto.getDiffieHellman #25366, limited to modp1) and use the same option/env variable to allow a revert to previous behavior
• update the docs to suggest avoiding sizes less than 2048 as suggested in
  Remove (or discourage) small DH groups in crypto.getDiffieHellman #25366

So the remaining issues to complete are:

• Remove (or discourage) small DH groups in crypto.getDiffieHellman #25366 (for 0.10.X and 0.12.X)
• tls: make server not use DHE in less than 1024bits for fix agaist Logjam Attack node#1739 - updated to have command line/env variable to back out. (Applies only to 0.12.X)

This issue will track getting those two done for the next release.