cmd/go/internal/lockedfile: add package and support library

lockedfile.File passes through to os.File, with Open, Create, and OpenFile
functions that mimic the corresponding os functions but acquire locks
automatically, releasing them when the file is closed.

lockedfile.Sentinel is a simplified wrapper around lockedfile.OpenFile for the
common use-case of files that signal the status of idempotent tasks.

lockedfile.Mutex is a Mutex-like synchronization primitive implemented in terms
of file locks.

lockedfile.Read is like ioutil.Read, but obtains a read-lock.

lockedfile.Write is like ioutil.Write, but obtains a write-lock and can be used
for read-only files with idempotent contents.

Updates #26794

Change-Id: I50f7132c71d2727862eed54411f3f27e1af55cad
Reviewed-on: https://go-review.googlesource.com/c/145178


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>