namespace change from bs_ to bsu_

motivation is that i want to retain the "Build Service" meaning
for bs_, see below.

bs_dloadurl is now bsu_dloadurl.

bs_project, bs_package are now bsu_bs_project and bsu_bs_package.
while they can be used in the upstream-defining bsu_dloadurl,
their meanings *are* Build Service project and package, resp.

i might introduce -U (upstreamm project) and -u (upstream package)
some time later, for symmetry.  or not, we'll see.

bs_tag -> bsu_tag

bs_version -> bsu_version

bs_tarball_hook -> bsu_tarball_hook
config variable tarball is now bsu_tarball

it's not yet directly user-settable, but i want to be ready.
expand, usage, $SELF defined after config files are read

i don't feel it's necessary to namespace these, and they
need not pollute dotfiles or risk redefinition.

while i'm at it, expand -> deref, see expand(1) for rationale.
tests: fake can be muted in .bs-update

BSU_QUIET_FAKE=1 in a test's .bs-update keeps tape.actual
empty; OTOH, the tests really need to use the mocks, otherwise
they're prone to spam third-party systems or screw filesystem
contents.
impl. detail: usage function user-friendlier
bs-update -t TARBALL => bs-update -d URL -t TARBALL

tests/170-option-dloadurl/, tests/330-no-option-dloadurl/ test -d
tests/200-option-tarball/, tests/320-no-option-tarball/ test -t
all tests have README files
all tests have .bs-update

all the newly added ones have BSU_QUIET_FAKE=1,
i *really* don't want to hammer the OBS with futile
requests (or worse).
commit message for osc(1) configurable

through bs-update -m "$text" or bsu_commitmsg in .bs-update.
defaults to the previously hardcoded value.
it is subject to deref.
tests for "missing optarg" states named as such
mv tests/190-option-n => tests/190-option-dryrun

it was the only option test named after the option letter,
the rest are named after the values the options represent.
tests: bs-update -n is tested after all other options

this test depends on those other options working correctly.
deref invocations w/ properly quoted operands

strips one line off the tape for each deref invocation.
bsu_dry_run is now bsu_dryrun

aligns the name with bsu_dloadurl and bsu_commitmsg.
tests/210-option-dryrun does not test deref
tests/450-dotfile/.bs-update loads tests/fakes

sets BSU_QUIET_FAKE (this test should not do network calls either).
bs-update -n / bsu_dryrun does not report pkgdir

it's not part of the interface and its initialization
is trivial.
config var specfile is now bsu_specfile

* bs-update -s foo.spec
* .bs-update support
* it is subject to deref
deref calls moved around a bit...

... so each config variable/value is fully set up in one place.
bsu_bs_package is subject to deref