Added support to list and remove orphaned packages.

A new target has been added to xbps-bin: autoremove. It works in
the same way than 'apt-get', but there's no stdin input. By default
it will list you all orphaned packages, and -f flag must be set
to really remove them.

The following is a real example for the git package, which requires
directly perl and curl-libs, and indirectly libidn and openssl.

$ xbps-bin -r ~/testing-xbps remove git
Removing package git-1.6.1.3 ... done.
$

$ xbps-bin -r ~/testing-xbps autoremove
The following packages were installed automatically
(as dependencies) and aren't needed anymore:

	perl-5.10.0 curl-libs-7.19.0 libidn-1.10 openssl-0.9.8j

If you are really sure you don't need them, use -f to confirm.
$

--HG--
extra : convert_revision : 70eebaa3d99be27753b94f580c8ec86330c7c2d3

lib/Makefile

@@ -9,8 +9,8 @@ LIBXBPS		= libxbps.so
 LIBXBPS_LDFLAGS	= -larchive -lprop -shared -Wl,-soname,$(LIBXBPS).$(MAJOR)
 
 OBJECTS	=	cmpver.o depends.o fexec.o humanize_number.o install.o
-OBJECTS	+=	plist.o remove.o repository.o requiredby.o sha256.o
-OBJECTS	+=	sortdeps.o unpack.o util.o
+OBJECTS	+=	orphans.o plist.o remove.o repository.o requiredby.o
+OBJECTS	+=	sha256.o sortdeps.o unpack.o util.o
 
 all: $(LIBXBPS)
 .PHONY: all