xbps-bin: added -f flag for remove target.

Now if any package is going to be removed and it's required by
other packages, it won't let you remove it unless -f is set.

Here's an example of how it looks like:

[juan@fedora-vm xbps]$ xbps-bin -r ~/testing-xbps remove glibc
WARNING! glibc is required by the following packages:

	zlib-1.2.3 ncurses-libs-5.7 gcc-libstdc++-4.3.2 e2fsprogs-libs-1.41.4
	cracklib-2.8.13 expat-2.0.1 ncurses-5.7 cpio-2.9
	module-init-tools-3.6 busybox-initramfs-1.13.2 udev-138 procps-3.2.7
	pam-1.0.2 dbus-libs-1.2.12 lzma-utils-libs-4.32.7 coreutils-7.1
	sed-4.1.5 grep-2.5.4 gawk-3.1.6 gzip-1.3.12
	bzip2-1.0.5 bash-4.0 less-424 gdbm-1.8.3
	groff-1.20.1 lzma-utils-4.32.7 dbus-1.2.12 proplib-0.3
	dash-0.5.4 findutils-4.4.0 util-linux-ng-2.14.2 initramfs-tools-0.92o
	file-5.00 diffutils-2.8.1 wget-1.11.4 man-db-2.5.3
	sysklogd-1.5 eject-2.1.5 shadow-4.1.2.2 sudo-1.7.0
	e2fsprogs-1.41.4 tzdata-2009a vim-7.2 upstart-0.5.1
	kernel-2.6.28.1 xbps-base-pkg-0.1 kbd-1.14.1

If you are sure about this, use -f to force deletion for this package.
[juan@fedora-vm xbps]$

--HG--
extra : convert_revision : eeb92925e51f11d5b3bf7e069ed4986ae5fb0c2d

doc/TODO

@@ -27,8 +27,6 @@ xbps-bin:
  * Add support to update packages.
  * While installing a package, check if version that is going to be
    installed is already installed.
- * While removing a package, abort if pkg is required by others
-   (looking at its requiredby array object).
 
 xbps-repo:
  * Replace binpkg-genindex.sh with a target for xbps-repo to generate the