make razor_set_update not loop forever on unsatisfiable dependencies

this is mostly a dead-end, as to get the next test working i need to
rewrite the code as suggested in the comment above razor_set_revalidate

redo razor_set_remove() to only do a single merge

Add support for removing packages

Should be redone to avoid rebuilding the set multiple times

First two tests pass now

fix rebuild_property_package_lists to put the lists in the right pool

(likewise rebuild_file_package_lists, which was copied from it)

Merge file lists when updating

Currently a bit of a mess due to repeated rewriting. To be cleaned up
after we decide whether or not to keep the current file layout

Fix file importing code to not generate some duplicate directories

build_file_tree() depends on the fact that the contents of a directory
are sorted immediately after it in the file list, but '.' sorts before '/'
lexicographically, so you'd actually end up with, eg:

/etc/yum
/etc/yum.conf
/etc/yum/pluginconf.d

which would cause a second entry for /etc/yum to be added to the file
list.

For now I've fixed this by make compare_filenames() do a special strcmp
by hand, manually sorting '/' before anything else.

redo test framework, with tests taken from yum sources

(none of them pass yet...)

misc fixes for edge cases

1) sets with only one package
2) packages with only one property
3) sets with only one property

remove some unused code

(trivial optimization)