go.sys/plan9: use syscall.ErrorString for errors

If we use a local type, it won't compare properly with errors from
the rest of the standard library. Errors are the one type from syscall
that propagates through the system, so it's important to have only
one type for them.
Will apply the corresponding change to the other packages once
this is approved.

Also delete some deprecated code. This is a new package; no need
to carry the past.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/126250043

plan9/syscall.go

@@ -17,20 +17,10 @@
 // For details of the functions and data types in this package consult
 // the manuals for the appropriate operating system.
 // These calls return err == nil to indicate success; otherwise
-// err represents an operating system error describing the failure.
+// err represents an operating system error describing the failure and
+// holds a value of type syscall.ErrorString.
 package plan9
 
-// StringByteSlice is deprecated. Use ByteSliceFromString instead.
-// If s contains a NUL byte this function panics instead of
-// returning an error.
-func StringByteSlice(s string) []byte {
-	a, err := ByteSliceFromString(s)
-	if err != nil {
-		panic("syscall: string with NUL passed to StringByteSlice")
-	}
-	return a
-}
-
 // ByteSliceFromString returns a NUL-terminated slice of bytes
 // containing the text of s. If s contains a NUL byte at any
 // location, it returns (nil, EINVAL).
@@ -45,11 +35,6 @@ func ByteSliceFromString(s string) ([]byte, error) {
 	return a, nil
 }
 
-// StringBytePtr is deprecated. Use BytePtrFromString instead.
-// If s contains a NUL byte this function panics instead of
-// returning an error.
-func StringBytePtr(s string) *byte { return &StringByteSlice(s)[0] }
-
 // BytePtrFromString returns a pointer to a NUL-terminated array of
 // bytes containing the text of s. If s contains a NUL byte at any
 // location, it returns (nil, EINVAL).