[release-branch.go1.2] misc/linkcheck: better redirect handling, use meaningful exit...

[release-branch.go1.2] misc/linkcheck: better redirect handling, use meaningful exit code

««« CL 14425049 / 093d299d669e
misc/linkcheck: better redirect handling, use meaningful exit code

Prevent linkcheck from following redirects that lead beyond the outside
the root URL.

Return a non-zero exit code when there are problems.

Some minor refactoring for clarity.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/14425049
»»»

R=golang-dev
CC=golang-dev
https://golang.org/cl/20360044