cmd/pkgsite: add -http flag
cmd/pkgsite: add -http flag

A -http flag is added which allows the user to specify which HTTP addr
to listen in on.

For golang/go#40371

Change-Id: Ibfe32281e9a821444df5e538fd7057f39318c546
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/290135
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite