tools: switch SELinux to runtime optional via dlopen

tools: switch SELinux to runtime optional via dlopen

Replace direct libselinux linking with runtime dlopen() using the
shared dl-utils infrastructure. All tools that use SELinux functions
now call them through selinux_call() macro and gracefully handle the
case when libselinux is not installed at runtime.

Affected tools: mkswap, chfn, chsh, lslogins, sulogin, vipw, namei,
mount, nsenter, login.

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