Show info about public key during login

This will show the key fingerprint and the associated comment, or
"password" when password was used, during login.

Eg.:

```
message":"User id: 1, logged in with: \"public_key:SHA256:FV3+wlAKGzYy7+J02786fh8N8c06+jga/mdiSOSPT7g:jo@desktop\",
```

or

```
message":"User id: 1, logged in with: \"password\",
...`

Signed-off-by: Jo Vandeginste <Jo.Vandeginste@kuleuven.be>

sftpd/internal_test.go

@@ -144,7 +144,7 @@ func TestUploadFiles(t *testing.T) {
 func TestWithInvalidHome(t *testing.T) {
 	u := dataprovider.User{}
 	u.HomeDir = "home_rel_path"
-	_, err := loginUser(u)
+	_, err := loginUser(u, "password")
 	if err == nil {
 		t.Errorf("login a user with an invalid home_dir must fail")
 	}