[client] Name the two config readers for what they do

ReadConfig and GetConfig differed in one thing — what happens when the file is
absent — and neither name said which was which:

- ReadConfig      -> ReadOrGenerateConfig  (reads it, or generates one in memory)
- GetConfig       -> GetExistingConfig     (reads it, or fails)

Three comments went with them:

- GetConfig's said "return with Config and if it was created. Errors out if it
  does not exist", which described a bool it does not return and a creation it
  never performs.
- ReadConfig's explained that it does not write, which is what a reader is
  supposed to do anyway.
- Server.getConfig's said it "errors out if it does not exist", which it does
  not — it resolves a default config, and now provisions the identity too.

client/server/setconfig_mdm_test.go

@@ -205,7 +205,7 @@ func TestSetConfig_MDMReject_AllOrNothing(t *testing.T) {
 
 	// Confirm RosenpassEnabled was NOT applied even though it was not
 	// in the conflict list: the request was rejected as a whole.
-	reloaded, err := profilemanager.GetConfig(cfgPath)
+	reloaded, err := profilemanager.GetExistingConfig(cfgPath)
 	require.NoError(t, err)
 	assert.False(t, reloaded.RosenpassEnabled, "non-conflicting field must not be applied when request is rejected")
 }