If the backend doesn't provide support for it, the matching is done in
refdb on top of a normal iterator.

Nobody should ever be using anything other than ALL at this level, so
remove the option altogether.

As part of this, git_reference_foreach_glob is now implemented in the
frontend using an iterator. Backends will later regain the ability of
doing the glob filtering in the backend.

This allows us to get a list of reference names in a loop instead of callbacks.

Create a new include/git2/sys/refs.h and move the reference alloc
functions there.  Also fix some documentation issues and some
minor code cleanups.