net/http, net/http/httputil: rename lk to mu

net/http, net/http/httputil: rename lk to mu

The conventional name for a sync.Mutex is "mu".

These "lk" names date back to a time before conventions.

Change-Id: Iee57f9f4423d04269e1125b5d82455c453aac26f
Reviewed-on: https://go-review.googlesource.com/21361
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>