spec: fix copy-here typo for "f3-modulize"

That wrong '$' escaping prevented arguments from passing into the python
script.
templates/common: fix '%pyver' definition, broken on RedHats

At Mandriva/Mageia, `%pyver` was already defined, thus masking a syntax
error. In RedHat, those macros failed to get defined, leaving `%pyver`
undefined and thus breaking "Requires: python(abi) = %pyver"
modulize.py: allow custom Jinja2 directories for SPEC templates

If a project needs to override the SPEC template, it shall be able to
define a custom (project-specific) directory of templates.
common.tmpl: add Jinja2 "blocks", so that parts can be overriden

Projects may need to customize parts of the spec, such as the %install,
%files or the SPEC prologue.