go.sys/unix: use syscall.Errno 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.

Ditto for syscall.Signal.

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/123490043

unix/syscall.go

@@ -17,21 +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 is an operating system error describing the failure.
-// On Unix systems, that error has type unix.Errno.
+// err represents an operating system error describing the failure and
+// holds a value of type syscall.Errno.
 package unix
 
-// 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("unix: 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).
@@ -46,11 +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).