os: add Seek whence constants

R=r, r2, rsc1, rsc, jacek.masiulaniec
CC=golang-dev
https://golang.org/cl/4344062
diff --git a/src/pkg/debug/elf/file.go b/src/pkg/debug/elf/file.go
index e69317a..60f913f 100644
--- a/src/pkg/debug/elf/file.go
+++ b/src/pkg/debug/elf/file.go
@@ -228,7 +228,7 @@
 	switch f.Class {
 	case ELFCLASS32:
 		hdr := new(Header32)
-		sr.Seek(0, 0)
+		sr.Seek(0, os.SEEK_SET)
 		if err := binary.Read(sr, f.ByteOrder, hdr); err != nil {
 			return nil, err
 		}
@@ -243,7 +243,7 @@
 		shstrndx = int(hdr.Shstrndx)
 	case ELFCLASS64:
 		hdr := new(Header64)
-		sr.Seek(0, 0)
+		sr.Seek(0, os.SEEK_SET)
 		if err := binary.Read(sr, f.ByteOrder, hdr); err != nil {
 			return nil, err
 		}
@@ -269,7 +269,7 @@
 	names := make([]uint32, shnum)
 	for i := 0; i < shnum; i++ {
 		off := shoff + int64(i)*int64(shentsize)
-		sr.Seek(off, 0)
+		sr.Seek(off, os.SEEK_SET)
 		s := new(Section)
 		switch f.Class {
 		case ELFCLASS32: