install: add --compare (-C) option to install file only when necessary

install: add --compare (-C) option to install file only when necessary

* src/install.c (have_same_content): New function to compare files
content.
(extra_mode): New function checking for non-permission bits in mode.
(need_copy): New function to check if copy is necessary.
(main): Handle new option --compare (-C).
(copy_file): Skip file copying if not necessary.
(usage): Show new option --compare (-C) in --help.
* tests/install/install-C: Basic tests for install --compare (-C).
* tests/install/install-C-root: Tests requiring root privileges.
* tests/install/install-C-selinux: Tests requiring SELinux.
* tests/Makefile.am: Add new tests for install --compare (-C).
* doc/coreutils.texi: Document new install option --compare (-C).
* NEWS: Mention the change.