os: don't consult Is methods on non-syscall error types

CL #163058 moves interpretation of platform-specific errors to the
syscall package. Package syscall errors implement an Is method which
os.IsPermission etc. consult. This results in an unintended semantic
change to the os package predicate functions: The following program
now prints 'true' where it used to print 'false':

	package main
	import "os"
	type myError struct{ error }
	func (e myError) Is(target error) bool { return target == os.ErrPermission }
	func main() { println(os.IsPermission(myError{})) }

Change the os package error predicate functions to only examine syscall
errors, avoiding this semantic change.

This CL does retain one minor semantic change: On Plan9, os.IsPermission
used to return true for any error with text containing the string
"permission denied". It now only returns true for a syscall.ErrorString
containing that text.

Change-Id: I6b512b1de6ced46c2f1cc8d264fa2495ae7bf9f5
Reviewed-on: https://go-review.googlesource.com/c/go/+/188817
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/os/error.go

@@ -115,7 +115,8 @@ func underlyingErrorIs(err, target error) bool {
 	if err == target {
 		return true
 	}
-	e, ok := err.(interface{ Is(error) bool })
+	// To preserve prior behavior, only examine syscall errors.
+	e, ok := err.(syscallErrorType)
 	return ok && e.Is(target)
 }