cp, mv, install: add SELinux support, but unlike with the Red Hat

cp, mv, install: add SELinux support, but unlike with the Red Hat

patch, mv and cp do not provide the "-Z context" option.
* src/copy.c: Include <selinux/selinux.h>.
(restore_default_fscreatecon): New function.
(copy_reg): Make cp --preserve=context work for existing destination.
(copy_internal): Likewise for new destinations.
* src/copy.h (cp_options) [preserve_security_context]: New member.
* src/cp.c: Include <selinux/selinux.h>.
(selinux_enabled): New global.
(usage): Mention new --preserve=context option.
(PRESERVE_CONTEXT): Define/use.
(decode_preserve_arg): Handle PRESERVE_CONTEXT.
(main): Remove an obsolete comment.
If --preserve=context is specified on a system without SELinux
enabled, give a diagnostic and fail.
* src/mv.c: Include <selinux/selinux.h>.
Set x->preserve_security_context if SELinux is enabled.
* src/install.c: Accept new "-Z, --context=C" option.
Accept --preserve-context option (but not -P option).
Accept alternate spelling: --preserve_context, for now.
Include <selinux/selinux.h> and "quotearg.h".
(selinux_enabled, use_default_selinux_context): New globals.
(PRESERVE_CONTEXT_OPTION): Define.
(cp_option_init): Default: do not preserve security context.
(setdefaultfilecon): New function.
(main): Honor new options.
* src/Makefile.am (mv_LDADD, cp_LDADD, ginstall_LDADD):
Add $(LIB_SELINUX).