build/vet: Fix all the shadowing reports with go1.6

Golang 1.6 is default version for the build now.

Additionally set 'GODEBUG=cgocheck=0' for now, until
we fix the erasure coding package.

Readmore here https://tip.golang.org/doc/go1.6#cgo

INSTALLGO.md

@@ -7,15 +7,15 @@ This installation document assumes Ubuntu 14.04+ on x86-64 platform.
 $ sudo apt-get install git build-essential
 ```
 
-##### Install Go 1.5+
+##### Install Go 1.6+
 
-Download Go 1.5+ from [https://golang.org/dl/](https://golang.org/dl/).
+Download Go 1.6+ from [https://golang.org/dl/](https://golang.org/dl/).
 
 ```sh
-$ wget https://storage.googleapis.com/golang/go1.5.1.linux-amd64.tar.gz
+$ wget https://storage.googleapis.com/golang/go1.6.linux-amd64.tar.gz
 $ mkdir -p ${HOME}/bin/
 $ mkdir -p ${HOME}/go/
-$ tar -C ${HOME}/bin/ -xzf go1.5.1.linux-amd64.tar.gz
+$ tar -C ${HOME}/bin/ -xzf go1.6.linux-amd64.tar.gz
 ```
 ##### Setup GOROOT and GOPATH
 
@@ -25,7 +25,7 @@ and GOPATH specifies the location of your project workspace.
 ```sh
 export GOROOT=${HOME}/bin/go
 export GOPATH=${HOME}/go
-export PATH=$PATH:${HOME}/bin/go/bin:${GOPATH}/bin
+export PATH=${HOME}/bin/go/bin:${GOPATH}/bin:$PATH
 ```
 ##### Source the new enviornment
 
@@ -71,7 +71,7 @@ and GOPATH specifies the location of your project workspace.
 export GOPATH=${HOME}/go
 export GOVERSION=$(brew list go | head -n 1 | cut -d '/' -f 6)
 export GOROOT=$(brew --prefix)/Cellar/go/${GOVERSION}/libexec
-export PATH=$PATH:${GOPATH}/bin
+export PATH=${GOPATH}/bin:$PATH
 ```
 ##### Source the new enviornment
 ```sh