http: add FileSystem interface, make FileServer use it

http: add FileSystem interface, make FileServer use it

Permits serving from virtual filesystems, such as files linked
into a binary, or from a zip file.

Also adds a gofix for:

http.FileServer(root, prefix) -> http.StripPrefix(prefix, http.FileServer(http.Dir(root)))

R=r, rsc, gri, adg, dsymonds, r, gri
CC=golang-dev
https://golang.org/cl/4629047