more key.c syncing

revision 1.114
date: 2013/12/29 04:20:04;  author: djm;  state: Exp;  lines: +17 -7;
to make sure we don't omit any key types as valid CA keys again,
factor the valid key type check into a key_type_is_valid_ca()
function

revision 1.113
date: 2013/12/29 02:49:52;  author: djm;  state: Exp;  lines: +2 -2;
correct comment for key_drop_cert()

revision 1.112
date: 2013/12/29 02:37:04;  author: djm;  state: Exp;  lines: +2 -2;
correct comment for key_to_certified()

revision 1.111
date: 2013/12/29 02:28:10;  author: djm;  state: Exp;  lines: +3 -2;
allow ed25519 keys to appear as certificate authorities

revision 1.110
date: 2013/12/07 00:19:15;  author: djm;  state: Exp;  lines: +2 -1;
set k->cert = NULL after freeing it