cmake: add the infrastructure for docbook-based man pages

Look for a docbook-to-man program as done in icecream, i.e
docbook2x-man first, then docbook-to-man and docbook2man.pl.

Add a install_docbook_man_page CMake macro to ease the generation and
installation process.
Add a simple man page for icemon
Merge pull request #9 from pinotree/man-page

Add a simple man page for icemon