qpkg: detect conflicts with installed packages

- prereq.c (installed_conflicts, free_conflicts, prereq): collect list of
  pre-existing conflicts in installed packages
- prereq.c (prereq): check if package being requested already conflicts
- test/instconf: regression test for conflicts with installed packages
- TODO: updated for full support of conflicts. Mention that we're not
  terribly efficient with handling conflicts.

qpkg/TODO

@@ -1,12 +1,8 @@
 Still left to do
 ================
 
-- check whether introducing a new package would cause a conflict
-
-  Update: conflicts among the packages considered for installation are now
-  checked.
-
-- compile the list of conflicts of installed packages
+- consider reducing the size of the lists of conflicts, e.g., by making
+  them unique via a red-black tree
 
 - handle Provides:
 
@@ -53,3 +49,10 @@ Done
   1172642 calls to comp_id, or 102% of the predicted "good case".
 
 - if there are multiple choices, try to prefer more recent versions
+
+- check whether introducing a new package would cause a conflict
+
+  Update: conflicts among the packages considered for installation are now
+  checked.
+
+- compile the list of conflicts of installed packages