Rename do_checkout to mksb, to distinguish it from do_checkout.sh.

"mksb" was the name of a tool which was part of the
OSF Development Environment for creating a "sandbox" environment for
doing development.

This name is also used elsewhere:
http://www.bsdcan.org/2011/schedule/attachments/156_BuildingBSD

The purpose of mksb is to prepare an environment for doing FreeNAS
development.  mksb does this by checking out the
sources from all the different source code repositories
which constitute a "sandbox development environment" for FreeNAS.
mksb will place the checked out sources in a hierarchy of directories
that the FreeNAS build system recognizes.

mksb will also write out any special config files into the sandbox
environment that are needed by the build system.
Rename .sb-vars to .sandbox-env
Change mksb so that it can be used in the following way:

  (1)  Invoke the tool with in its simplest form:
         ./mksb -n testdir1

  (2)  mksb will create a directory "testdir1" and
       do the initial checkout of freenas.git into a testdir1/freenas
       directory.

  (3)  mksb will write out some config files into the testdir1 "sandbox".
       This includes config files like .git-repo-setting
       which specify INTERNAL or EXTERNAL.

  (4)  mksb will chdir to "testdir1/freenas" and execute
       "make checkout"

       This will do the second-level checkout which

When mksb has completed, the user can:

   cd testdir1/freenas
   make release

The idea is to keep 'mksb' as a simple bootstrap tool which
does the initial "git clone" followed by a "make checkout".
The logic for actually checking out a full sandbox should
be kept out of mksb as much as possible.  That way, mksb
can live outside the sandbox, and not change very often.

The logic for checking out a sandbox can change more frequently,
as long as "make checkout" is the command for checking out the sandbox.