[release-branch.go1.9] all: merge master into release-branch.go1.9

0f5d61c4 imports: print dir of candidates in addition to import path
a237aba5 godoc: fix out-of-bounds panic when serving top-level files
4e70a1b2 godoc: add GoogleCN property to pages
fcc44a63 cmd/getgo: add a user-agent to download requests
3fd990c6 cmd/tip: fix the build
d07a458d cmd/tip: add a cert cache, clean up Kubernetes config, use update-deps
9badcbe4 cmd/getgo: prompt warning if an earlier installation exists
6fdd948b godoc: remove disabled admin code
f2b3bb00 cmd/getgo: fix builds
001b4ec8 cmd/getgo: have consistent messages
29518d98 cmd/getgo: display that -i stands for interactive mode
5724bdc2 x/tools/godoc: fix redirect to Gerrit
ac1e4b19 cmd/getgo: initial commit

Change-Id: Ie58293a2621bbabbadea4f9431c6fe7bc4aa329f