os, syscall: implement ReadAt for unix

Windows will take more work, so test is skipped there.

src/os/file_unix.go

@@ -2,7 +2,10 @@
 
 package os
 
-import "syscall"
+import (
+	"io"
+	"syscall"
+)
 
 type syscallFd = int
 
@@ -22,3 +25,16 @@ func Pipe() (r *File, w *File, err error) {
 	}
 	return
 }
+
+// ReadAt reads up to len(b) bytes from the File starting at the given absolute offset.
+// It returns the number of bytes read and any error encountered, possibly io.EOF.
+// At end of file, Pread returns 0, io.EOF.
+// TODO: move to file_anyos once ReadAt is implemented for windows
+func (f unixFileHandle) ReadAt(b []byte, offset int64) (n int, err error) {
+	n, err = syscall.Pread(syscallFd(f), b, offset)
+	err = handleSyscallError(err)
+	if n == 0 && err == nil {
+		err = io.EOF
+	}
+	return
+}