sftpd: add support for SSH user certificate authentication

This add support for PROTOCOL.certkeys vendor extension:

https://cvsweb.openbsd.org/src/usr.bin/ssh/PROTOCOL.certkeys?rev=1.8

Fixes #117

Signed-off-by: Nicola Murino <nicola.murino@gmail.com>

config/config.go

@@ -61,6 +61,7 @@ func init() {
 			KexAlgorithms:           []string{},
 			Ciphers:                 []string{},
 			MACs:                    []string{},
+			TrustedUserCAKeys:       []string{},
 			LoginBannerFile:         "",
 			EnabledSSHCommands:      sftpd.GetDefaultSSHCommands(),
 			KeyboardInteractiveHook: "",