pstree: add apparmor support

pstree: add apparmor support

While the current fallback method might obtain the correct AppArmor
context by checking /proc/self/attr/current, it is not guaranteed that
this value will be the context attributed by AppArmor. The current
interface being used upstream is /proc/self/attr/apparmor/current, and
that can be obtained by using the AppArmor library functions.

In order to avoid link time dependencies, we are loading the apparmor
library dynamically, just like is currently done by SELinux.

Signed-off-by: Georgia Garcia <georgia.garcia@canonical.com>