github.com/cznic/fileutil: fix FreeBSD build.

Fixes build broken in
https://camlistore.org/gw/01ce3859291e9e012b497af13a4c2dd82d2d1d4f

Change-Id: Ieede30ab41c6896f0b9964895ca7237fa6c34b5a
pkg/osutil: add Lock function and tests.

Prep for using cznic/kv in camput and as index.Storage.

Change-Id: If912f7cb6b33247217f7812ac2082414436c0bf5
Merge branch 'master' of https://camlistore.googlesource.com/camlistore