Fix minor typos found by codespell

This patch fixes some minor typos in comments and strings found by
codespell.

While at it, also expand some variable names that were not typos, but
caused false positives, and end up being more readable anyway.

internal/courier/smtp_test.go

@@ -58,7 +58,7 @@ func TestSMTP(t *testing.T) {
 	// Put a non-existing host first, so we check that if the first host
 	// doesn't work, we try with the rest.
 	// The host we use is invalid, to avoid having to do an actual network
-	// lookup whick makes the test more hermetic. This is a hack, ideally we
+	// lookup which makes the test more hermetic. This is a hack, ideally we
 	// would be able to override the default resolver, but Go does not
 	// implement that yet.
 	testMX["to"] = []*net.MX{