Revert "coverity-detected bug: don't free "/""

This reverts commit 5cad2f04bec614a84e872f68697b5092a409d5a1.
Fix invalid free() in lxc_global_config_value().

Commit 4878dac496543dae0a26f5c1156bbfda63c8ea62 introduced possible
incorrect usage of free().
Also remove unneeded strdup().

Signed-off-by: Andrey Mazo <mazo@telum.ru>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>