all: Fix some linter errors (#5499)

I'm working through linter complaints, these are some fixes. Broad
categories:

1) Ignore errors where we can ignore errors: add "_ = ..." construct.
you can argue that this is annoying noise, but apart from silencing the
linter it *does* serve the purpose of highlighting that an error is
being ignored. I think this is OK, because the linter highlighted some
error cases I wasn't aware of (starting CPU profiles, for example).

2) Untyped constants where we though we had set the type.

3) A real bug where we ineffectually assigned to a shadowed err.

4) Some dead code removed.

There'll be more of these, because not all packages are fixed, but the
diff was already large enough.

cmd/syncthing/mocked_config_test.go

@@ -30,7 +30,7 @@ func (c *mockedConfig) LDAP() config.LDAPConfiguration {
 
 func (c *mockedConfig) RawCopy() config.Configuration {
 	cfg := config.Configuration{}
-	util.SetDefaults(&cfg.Options)
+	_ = util.SetDefaults(&cfg.Options)
 	return cfg
 }