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/test/instconf

@@ -0,0 +1,50 @@
+#!/bin/sh
+. ./Common
+
+###############################################################################
+
+qpkg_fail "installed package conflicts with dependency" prereq A <<EOF
+Package: B
+Version: 0
+Architecture: test
+Filename: B
+
+Package: A
+Version: 0
+Architecture: test
+Depends: B
+Filename: A
+
+Package: C
+Version: 0
+Architecture: test
+Conflicts: B
+Status: installed
+EOF
+expect <<EOF
+can't resolve A
+EOF
+
+###############################################################################
+
+qpkg_fail "installed package conflicts with initial package" prereq A <<EOF
+Package: B
+Version: 0
+Architecture: test
+Filename: B
+
+Package: A
+Version: 0
+Architecture: test
+Depends: B
+Filename: A
+
+Package: C
+Version: 0
+Architecture: test
+Conflicts: A
+Status: installed
+EOF
+expect <<EOF
+A conflicts with installed packages
+EOF