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:
diff --git a/src/pkg/debug/pe/file.go b/src/pkg/debug/pe/file.go
index 1bcbdc5..b99131e 100644
--- a/src/pkg/debug/pe/file.go
+++ b/src/pkg/debug/pe/file.go
@@ -132,7 +132,7 @@
 	} else {
 		base = int64(0)
 	}
-	sr.Seek(base, 0)
+	sr.Seek(base, os.SEEK_SET)
 	if err := binary.Read(sr, binary.LittleEndian, &f.FileHeader); err != nil {
 		return nil, err
 	}
@@ -140,7 +140,7 @@
 		return nil, os.NewError("Invalid PE File Format.")
 	}
 	// get symbol string table
-	sr.Seek(int64(f.FileHeader.PointerToSymbolTable+18*f.FileHeader.NumberOfSymbols), 0)
+	sr.Seek(int64(f.FileHeader.PointerToSymbolTable+18*f.FileHeader.NumberOfSymbols), os.SEEK_SET)
 	var l uint32
 	if err := binary.Read(sr, binary.LittleEndian, &l); err != nil {
 		return nil, err
@@ -149,9 +149,9 @@
 	if _, err := r.ReadAt(ss, int64(f.FileHeader.PointerToSymbolTable+18*f.FileHeader.NumberOfSymbols)); err != nil {
 		return nil, err
 	}
-	sr.Seek(base, 0)
+	sr.Seek(base, os.SEEK_SET)
 	binary.Read(sr, binary.LittleEndian, &f.FileHeader)
-	sr.Seek(int64(f.FileHeader.SizeOfOptionalHeader), 1) //Skip OptionalHeader
+	sr.Seek(int64(f.FileHeader.SizeOfOptionalHeader), os.SEEK_CUR) //Skip OptionalHeader
 	f.Sections = make([]*Section, f.FileHeader.NumberOfSections)
 	for i := 0; i < int(f.FileHeader.NumberOfSections); i++ {
 		sh := new(SectionHeader32)