[master] low priority housekeeping patches

[David Cantrell]

1) Remove isys cpio code and use libarchive in loader.
2) Add in some libarchive helper functions to loader/unpack.c.
3) Make use of the libarchive helper functions in rpmextract.c.
4) Remove mkdirChain() and related functions in favor of
   g_mkdir_with_parents() in glib.
5) Removal of various other unused related bits of code.

I'll test these at some point in the future (probably this coming
weekend), but just wanted to toss them out now for people to look
over.