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>