Fix some broken sourceforge.net links

Looks like there's issues with sourceforge project
pages. Given that sourceforge isn't really what
it used to be, trying to find alternative URLs
where possible.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

docs/admin/b2d_volume_resize.md

@@ -73,7 +73,7 @@ The `boot2docker` command reads its configuration from the `$BOOT2DOCKER_PROFILE
 
 This solution increases the volume size by first cloning it, then resizing it
 using a disk partitioning tool. We recommend
-[GParted](http://gparted.sourceforge.net/download.php/index.php). The tool comes
+[GParted](https://sourceforge.net/projects/gparted/files/). The tool comes
 as a bootable ISO, is a free download, and works well with VirtualBox.
 
 1. Stop Boot2Docker
@@ -102,7 +102,7 @@ as a bootable ISO, is a free download, and works well with VirtualBox.
 
 5. Download a disk partitioning tool ISO
 
-  To resize the volume, we'll use [GParted](http://gparted.sourceforge.net/download.php/).
+  To resize the volume, we'll use [GParted](https://sourceforge.net/projects/gparted/files/).
   Once you've downloaded the tool, add the ISO to the Boot2Docker VM IDE bus.
   You might need to create the bus before you can add the ISO.
 

docs/reference/builder.md

@@ -572,7 +572,7 @@ The cache for `RUN` instructions can be invalidated by `ADD` instructions. See
   For systems that have recent aufs version (i.e., `dirperm1` mount option can
   be set), docker will attempt to fix the issue automatically by mounting
   the layers with `dirperm1` option. More details on `dirperm1` option can be
-  found at [`aufs` man page](http://aufs.sourceforge.net/aufs3/man.html)
+  found at [`aufs` man page](https://github.com/sfjro/aufs3-linux/tree/aufs3.18/Documentation/filesystems/aufs)
 
   If your system doesn't have support for `dirperm1`, the issue describes a workaround.
 

docs/security/security.md

@@ -51,7 +51,7 @@ common Ethernet switch; no more, no less.
 How mature is the code providing kernel namespaces and private
 networking? Kernel namespaces were introduced [between kernel version
 2.6.15 and
-2.6.26](http://lxc.sourceforge.net/index.php/about/kernel-namespaces/).
+2.6.26](http://man7.org/linux/man-pages/man7/namespaces.7.html).
 This means that since July 2008 (date of the 2.6.26 release
 ), namespace code has been exercised and scrutinized on a large
 number of production systems. And there is more: the design and