lxc_container: Check retval of capng_get_caps_process()

lxc_container: Check retval of capng_get_caps_process()

Added in v0.6.5~14 the call to capng_get_caps_process() inside of
lxcContainerDropCapabilities() is not really explained in the
commit message. But looking into the libcap-ng sources it's to
initialize the internal state of the library.

But with recent libcap-ng commit [1] (which some bleeding edge
distros - like Fedora rawhide - already picked up) the function
has been marked as 'warn unused result'. Well, check for its
retval then.

1: https://github.com/stevegrubb/libcap-ng/commit/a0743c335c9a16a2fda9b25120a5523742119e47

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>