fix a few type errors, make ErrorString a value

will submit with fixed compiler

R=r
DELTA=2  (0 added, 0 deleted, 2 changed)
OCL=28371
CL=28379

src/lib/os/error.go

@@ -14,8 +14,8 @@ type Error interface {
 // A helper type that can be embedded or wrapped to simplify satisfying
 // Error.
 type ErrorString string
-func (e *ErrorString) String() string {
-	return *e
+func (e ErrorString) String() string {
+	return e
 }
 
 // Errno is the Unix error number.  Names such as EINVAL are simple