Support semanage.mods

Aimee OS variants can now supply a `semange.mods` file in their
configuration directories.  This file contains SELinux customizations,
in the format produced by `semanage export`.  Variants can use this
mechanism to set SELinux booleans, change user/login mappings, and other
SELinux policy customization.  At build time, the base `selinux.mods`
file in the source directory is combined with the variant's file, if it
exists, and the resultant customizations are applied in a single
transaction.

semanage.mods

@@ -0,0 +1,15 @@
+boolean -D
+login -D
+interface -D
+user -D
+port -D
+node -D
+fcontext -D
+module -D
+ibendport -D
+ibpkey -D
+permissive -D
+boolean -m -1 ssh_sysadm_login
+boolean -m -1 systemd_tmpfiles_manage_all
+login -m -s root -r 's0-s0:c0.c1023' root
+user -m -L s0 -r s0-s0:c0.c1023 -R 'sysadm_r' root