cmd/cgo: Add support for C function pointers

* Add a new kind of Name, "fpvar" which stands for function pointer variable
* When walking the AST, find functions used as expressions and create a new Name object for them
* Track functions which are only used in expr contexts, and avoid generating bridge code for them

R=golang-dev, minux.ma, fullung, rsc, iant
CC=golang-dev
https://codereview.appspot.com/9835047

Committer: Russ Cox <rsc@golang.org>
net: implement DNS TCP fallback query if UDP response is truncated

Fixes issue 5686.

R=golang-dev, bradfitz, mikioh.mikioh
CC=golang-dev
https://codereview.appspot.com/12458043

Committer: Brad Fitzpatrick <bradfitz@golang.org>