complex tag->version transforms using a hook

$bsu_version is derived from $bsu_tag using bsu_version_hook
if that function exists.
README.rest synced with man page, reality
osc(1) abstracted away from "main"...

... to bring it to the level of abstraction of the rest of it.
-b causes osc build just before osc ci
-C causes skipping of osc ci
-d . uses git archive

getting closer to comfortable bs-update-centered workflow:

  > git clone ... mypackage
  > cd mypackage
  > edit
  > git ci
  > bs-update -Cbd . HEAD x.y.z

if all went well, one could go on with

  > git tag -a vx.y.z
  > bs-update vx.y.z

the major missing piece ATM is the "x.y.z" argument above in

  > bs-update -Cbd . HEAD x.y.z

(it would set $bsu_version); we have bsu_version_hook() but
that is suboptimal for this use-case: it's too much work to
put in place compared to adding an argument, and needs to be
disabled again before dropping -C.
usage mentions two operands (<tag> [<version>])
bs-update's second argument sets $bsu_version

$bsu_version is set using the first good of these sources:

* $2
* bsu_version_hook
* $bsu_tag
bs-update <tag> <version> tested
rpmlint complains about trailing dot in Summary
rpmlint complains about nonstandard package group
manpage: FILES/Variables updated

* bsu_version cannot be set in config files
* bsu_bs_commit, bsu_dryrun, bsu_test_build documented
manpage revamp

* option descriptions do not mention config variables;
  it was awkward
* bsu_version is internal, no mentions
manpage: example is more realistic now
manpage: FILES/Variables section takes less space
README.rest and manpage synced again
manpage: typo
README.rest: markup changes

docutils' option list renders optargs using slanted
fixed-width font; it appears italics and fixed-width
cannot be combined using inline markup; so far i opted
to have them rendered with straight fixed-width.
this was prone to confusion re what is a placeholder
vs. an actual literal, so i'm going for italics instead.
specfile: PREFIX is %{_prefix} instead of "/usr"

i might want to build an rpm for FreeBSD you know. /jk
while i'm not aware of any rpm-using OS with a different value,
this bit of flexibility does not hurt.
specfile: make all runs in the %build script

where it belongs.