Integrate the functionality from runscript_selinux.so

runscript used to dlopen() runscript_selinux.so. This adds equivalent
functionality directly in to runscript instead. It authenticates with
either PAM or shadow and optionally has a dep on audit.

X-Gentoo-Bug: 517450
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=517450

src/rc/rc-selinux.h

@@ -30,6 +30,6 @@ int selinux_util_open(void);
 int selinux_util_label(const char *path);
 int selinux_util_close(void);
 
-void selinux_setup(int argc, char **argv);
+void selinux_setup(char **argv);
 
 #endif