add ECC strings to ciphers(1), point out difference between DH and ECDH

 * Make a clear distinction between DH and ECDH key exchange.
 * Group all key exchange cipher suite identifiers, first DH then ECDH
 * add descriptions for all supported `*DH*` identifiers
 * add ECDSA authentication descriptions
 * add example showing how to disable all suites that offer no
   authentication or encryption