Add support for 'install --target-directory', an option

Add support for 'install --target-directory', an option
that has been documented for years but not implemented (!).

(TARGET_DIRECTORY_OPTION): New var.
(long_options, main, usage): Add --target-directory.
(target_directory_operand): New function, stolen from mv.c.
(main): Use it.  Check for -d and --target-directory.
Alter wording of diagnostics to match other programs.