Fix improper handling of repo_groups sections

Fix improper handling of repo_groups sections

When parsing repo_groups ini entries, repo_shell was calling
strip_repo() on the entry value, then calling str_has_word() to see if
the desired repo was in the list of words represented by the ini value.
Of course this is wrong, each token in the value string must have
strip_repo() called upon it instead.

The solution:

* Move strip_repo() to stringutils.c
* Add str_has_repo(), when can call strip_repo() on each token in its
  string.