lxc: remove dependency on headers >= 3.0

lxc: remove dependency on headers >= 3.0

Now that libcap no longer needs kernel headers >= 3.0, we can remove
this dependency from lxc. However, building with headers 2.6.32
exhibits a build issue caused by the redefinition of the setns()
function.

Since setns() is not implemented in the C library, lxc provides its
own version. However, for some reason, while the C library doesn't
implement setns(), it provides a prototype for it, which is not
exactly the same as the one in lxc, causing a build failure. We re-use
a solution implemented in gdb to solve the same problem: define in lxc
a function called do_setns(), which calls setns() when available, or
manually does the system call otherwise.

Of course, with old kernels the system call will not be available, so
things will fail at runtime, but this was anyway already the behavior
of lxc's setns() dummy implementation.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>