copy,install: avoid unnecessary security context translations

copy,install: avoid unnecessary security context translations

Do not perform SELinux context translation for operations not involving
user input or output.  Context translation converts MCS/MLS labels into
human readable form, which is useful for user facing applications like
ls(1) or the --context=CTX argument of cp(1).

* src/copy.c (set_process_security_ctx): Use raw selinux variants.
* src/install.c (need_copy): Likewise.
(setdefaultfilecon): Likewise.
* src/selinux.c (computecon): Likewise.
(defaultcon): Likewise.
* tests/cp/no-ctx.sh: Add raw variants to preload lib.
* NEWS: Mention the improvement.