This script can be used to start/stop containers during rbm builds.

rbm_tmp_dir is a directory inside tmp_dir that is automatically removed
when rbm exits.

We also fix an issue in urlget when used within the exec script of an
input file. In this case the input_files_id was depending on the value
of tmp_dir because it is used in urlget. We fix that be setting
getting_input_files_id when computing input_files ids, so that urlget
can avoid using tmp_dir in this case.

For simplification, we remove some unused features. Those features can
still be reimplemented externally if needed.

We remove the following options:
 - version_command
 - rpm
 - srpm
 - rpmspec
 - rpm_rel
 - debian_create
 - deb
 - deb_src
 - use_pbuilder
 - debian_revision
 - options

And the following commands:
 - rpmspec
 - rpm
 - srpm
 - deb-src
 - deb
 - pkg
 - publish

Exit with an error message if target, target_prepend or target_append
defined in input_files is not an array.

When using the build command, step gets initialized to 'build'. However
this was not done in some of the other non-build commands where the
value of step was left to rbm_init.

Instead, use the sha256file functon, which keeps the sha256sum of
files to avoid computing it twice. The function can now take a HASH as a
second argument, which can contain the key remove_cache indicating that
the sha256sum of a file should be cleared: we use this when we
find that a file has the wrong checksum and needs to be downloaded
again.