Gentoo: Rework Gentoo ebuild into overlay

This commit squashes all the little changes from the previous
integration branch into one, adjusts to the new Makefile changes, and
introduces an overlay layout so that the contrib/gentoo-bees subtree
can be directly added as a Portage overlay to the system.

The following list contains the previous commit descriptions:

sys-fs/bees: Keyword tested architecture ~amd64

    Bees was tested on this platform.

sys-fs/bees: Add kernel version checks

    Add checking the kernel versions and write some info and/or warnings
    before building and installing the package. Running bees on older
    kernels may have some serious performance and stability impacts, let's
    tell the user about it.

    Closes #55

sys-fs/bees: Add metadata.xml

sys-fs/bees: There's no configure script

    So, there's no point in calling "default".

sys-fs/bees: Simplify src_configure()

sys-fs/bees: Don't depend on markdown

    It makes no sense to install both README.md and README.html, and we can
    get rid of one dependency.

Dependencies: btrfs-progs is no longer a buildtime-only dep

    It is actually needed by the bees service wrapper script, as pointed out
    by Gentoo QA review.

sys-fs/bees: DOCS is not needed

    "COPYING" is already covered by the licensing. The ebuild defaults
    already include README*

sys-fs/bees: Make warnings exclusive

    It was recommended by Gentoo QA to show only either one or another
    warning, and change the texts accordingly.

sys-fs/bees: RDEPEND is not implicit

    RDEPEND does not implicitly default to DEPEND. Let's explicitly set the
    variable.

sys-fs/bees: IUSE=test is only needed for explicit dependencies

    Thus, remove it.

Signed-off-by: Kai Krakow <kai@kaishome.de>

README.md

@@ -417,7 +417,7 @@ Dependencies
 * btrfs-progs (tested with 4.1..4.15.1)
 
   Needed for btrfs.h and ctree.h during compile.
-  Not needed at runtime.
+  Also needed by the service wrapper script.
 
 * libuuid-dev