libmount: use shared dl-utils for SELinux dlopen

libmount: use shared dl-utils for SELinux dlopen

Add lib/dl-selinux.c and include/dl-selinux.h as a SELinux dlopen
wrapper using the shared dl-utils infrastructure. This replaces the
direct libselinux linking with runtime dlopen(), allowing libmount to
gracefully handle the case when libselinux is not installed.

The hook_selinux.c and lib/selinux-utils.c now use selinux_call()
macro to call SELinux functions through the dlopen'd function pointer
table.

Signed-off-by: Karel Zak <kzak@redhat.com>