Make lxc-checkconfig work with kernel versions > 3

Make lxc-checkconfig work with kernel versions > 3

(1) Add test for kernel version greater 3.
(2) Use && and || instead of -a and -o as suggested in
    http://www.unix.com/man-page/posix/1p/test/.

lxc-checkconfig will currently report "missing" on "Cgroup memory controller"
for kernel versions greater 3. This happens because the script, before checking
for the corresponding memory variable in the kernel config, currently will test
whether we have a major kernel version greater- or equal to 3 and a minor kernel
version greater- or equal to 6. This adds an additional test whether we have a
major kernel version greater than 3.

Signed-off-by: Christian Brauner <christianvanbrauner@gmail.com>