Proposal: add --reflink to install

[Mingye Wang]

It seems logical to add a --reflink option to the install(1) utility
since it, like cp, also copies files. Enabling such an option would
theoretically speed up `make install` in quite some cases.

A related problem is whether reflink=auto should be default for
`install`, since `install` does not tend to have new flags added and
Makefile writers don't usually add a INSTALLFLAGS variable to handle
them. To truly take advantage of such a change making it default may
be necessary. Personally I believe that the argument against using
reflink by default in cp does not apply to this case[1], as nobody
uses `install` to backup files.

  [1]: https://unix.stackexchange.com/a/152639/73443