go/types: callback for *ast.Ident -> Object mapping

Also re-enabled resolver test.

R=adonovan
CC=golang-dev
https://codereview.appspot.com/7107043
go/types: mark completely imported packages as such

R=adonovan
CC=golang-dev
https://codereview.appspot.com/7103055
time: fix race

Fixes issue 4622

R=golang-dev, dave, dvyukov
CC=golang-dev
https://codereview.appspot.com/7103046
go/types: various minor fixes

- always set the Pkg field in QualifiedIdents
- call Context.Ident for all identifiers in the AST that denote
  a types.Object (bug fix)
- added test that Context.Ident is called for all such identifiers

R=adonovan
CC=golang-dev
https://codereview.appspot.com/7101054
go/types: set type of lhs ident in type switch guards

(bug fix)

R=adonovan
CC=golang-dev
https://codereview.appspot.com/7098059
syscall: simplify socket control messages

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7016044
net: simplify ListenMulticastUDP

R=rsc, iant, dave
CC=golang-dev
https://codereview.appspot.com/6999053
doc: fix various fragment links

R=dsymonds
CC=golang-dev
https://codereview.appspot.com/7094058