Bug#398404: branch created for the pbuilder mode option

[Osamu Aoki]

Hi, 

(CCed to bug reporters of related bugs: 384031, 373807)

I finally got to create branch for mode option.  It is "osamu" and
merged from the latest Loic's commit to "master" now.  It has been
committed as branch to the pbuilder git repo.

The optional mode argument, e.g. "--mode mode1", is a way to support
multiple build environments. It creates pbuilder working environment
where each mode, e.g. "mode1", keeps all its data (chroot, build
results, aptcache, ...) under /var/cache/pbuilder/mode1/ .

If we attempt to support complicated chroot configurations from command
line options, it becomes too complicated and hard to learn.  It will be
unmaintainable too.  We can see some glimpses with the support of
experimental.  People may ask what about security support of testing,
volatile, .....  This mode option solves these problems.

Keep it simple and stupid.

The work flow with the mode option, e.g. "mode1" goes as:

$ sudo pbuilder --mode mode1 --create
$ sudo pbuilder --mode mode1 --login --save-after-login
... interactive configuration of chroot
^D
$ pdebuild -- --mode mode1

Alternatively, you may use --no-targz instead of --save-after-login .
(these are tested)

If any automatic configuration is needed, I guess use --exec instead of
--login with some hook script. (this is not tested)

This is a clean up and minimal change version of my old proposal.

Commit is in 2 steps.
 1st. critical changes
 2nd. associated changes

I admit it requires change to cowdancer if cowdancer also want to use
this --mode thing.  I also admit this needs a lot of testing.

Unlike last time I discussed, this is current and usable.

I hope this is interesting to you.

Good night.
                       Osamu