pushing: add new can_user_push_http utility

pushing: add new can_user_push_http utility

In similar fashion to the can_user_push utility, push permissions
can be checked.  This version of the utility reads a group file
directly and so can be used to check http push permissions whereas
the can_user_push utility always uses the getgrnam interface and
so can only check the system's (or chroot's) group file.

While the Girocco::Project can_user_push function can be used to
do the same checking as this utility, this utility avoids the
need to load the Perl interpreter just to check push permissions.