libyelp: remove unused variables

[endlessm/eos-shell#4234]
libyelp: fix unsigned char vs char pointers

libxml2 uses unsigned chars, but glib used signed chars. Add a few casts
in the right places to make GCC happy.

[endlessm/eos-shell#4234]
mallard-document: add missing include

[endlessm/eos-shell#4234]
yelp-transform: remove unused function declarations

[endlessm/eos-shell#4234]
docbook-document: use correct order for arguments to for() loop

Whoops... I wonder how many bugs this was causing!

[endlessm/eos-shell#4234]
Merge pull request #21 from endlessm/shell/4234

Shell/4234