Added support to verify reverse dependencies in a transaction.

See https://plus.google.com/106812073015308749022/posts/ZXMHimd8iKd

lib/Makefile

@@ -38,6 +38,7 @@ OBJS += package_remove.o package_find_obsoletes.o package_state.o
 OBJS += package_unpack.o package_register.o package_script.o
 OBJS += transaction_commit.o transaction_package_replace.o
 OBJS += transaction_dictionary.o transaction_sortdeps.o transaction_ops.o
+OBJS += transaction_revdeps.o
 OBJS += download.o initend.o pkgdb.o package_conflicts.o
 OBJS += plist.o plist_find.o plist_match.o archive.o
 OBJS += plist_remove.o plist_fetch.o util.o util_hash.o