... so that we don't have to care if other modules have already created
it (as long as our path inside it is unique)

All this module does is create a temp directory FFOO_TMP, that can be
shared among modules.  This simplifies clenanup but should be only
used very rarely and only by modules (i.e. not by user code).

 *  Support `date`'s +FORMAT notation.
 *  Use FFOO_TESTING_ARTIFACTS if argument not provided

Debug calls with command line to be executed are dangerously easy
to become obsolete and misinforming.  They should be only used ad-
hoc for a very short time (i.e. when debiugging the command), or
replaced by construct such as

    cmd="some command --with args | maybe pipe"
    debug -v cmd
    $cmd

provided it's safe.

Semver is implied for all forms of version information, not just the
particular one.

Guess is now section head + FFOO_INI_SUFFIX, to allow for customized
config trees.

The (not sure if )written convention is that module's variable names
should be composed as

    FFOO_BAR_BAZ

for variable bearing *baz* in module named *bar*.

The stack now supports passing down parameters to yum, so there's no
need to another case.

Allow passing doen yum arguments like `--enablerepo="*"`