net/http: prepend ./ to directory list hrefs in FileServer

net/http: prepend ./ to directory list hrefs in FileServer

Certain browsers (Chrome 53, Safari 9.1.2, Firefox 46) won't correctly
follow a directory listing's links if the file name begins with a run
of characters then a colon, e.g. "foo:bar". Probably mistaking it for
a URI. However, they are happy to follow "./foo:bar", so this change
prepends "./" to all link hrefs in the directory listing of
FileServer.

Change-Id: I60ee8e1ebac73cbd3a3ac0f23e80fdf52e3dc352
Reviewed-on: https://go-review.googlesource.com/27440
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>