install: new option: --strip-program,

install: new option: --strip-program,

...to specify the program used to strip binaries
* src/install.c (main): Handle new option --strip-program.
(strip): Use strip program from global variable strip_program.
(usage): Mention new option --strip-program in --help.
* tests/tests/strip-program: Test case for new option --strip-program.
* tests/Makefile.am: Add new test case to test set.
* doc/coreutils.texi: Mention new option --strip-program.
* NEWS: Mention the change.
* TODO: Remove completed task.