Switch from x/net/context to context

Since Go 1.7, context is a standard package, superceding the
"x/net/context". Since Go 1.9, the latter only provides a few type
aliases from the former. Therefore, it makes sense to switch to the
standard package.

This commit was generated by the following script (with a couple of
minor fixups to remove extra changes done by goimports):

	#!/bin/bash

	if [ $# -ge 1 ]; then
		FILES=$*
	else
		FILES=$(git ls-files \*.go | grep -vF ".pb.go" | grep -v
	^vendor/)
	fi

	for f in $FILES; do
		printf .
		sed -i -e 's|"golang.org/x/net/context"$|"context"|' $f
		goimports -w $f
		awk '	/^$/ {e=1; next;}
			/[[:space:]]"context"$/ {e=0;}
			{if (e) {print ""; e=0}; print;}' < $f > $f.new && \
				mv $f.new $f
		goimports -w $f
	done
	echo

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

cmd/ctr/commands/shim/shim.go

@@ -19,12 +19,11 @@
 package shim
 
 import (
+	gocontext "context"
 	"fmt"
 	"io/ioutil"
 	"net"
 
-	gocontext "context"
-
 	"github.com/containerd/console"
 	"github.com/containerd/containerd/cmd/ctr/commands"
 	shim "github.com/containerd/containerd/linux/shim/v1"