chown does not support `--reference X` syntax, only `--reference=X`

[julian-klode]

First reported in https://bugs.launchpad.net/ubuntu/+source/rust-coreutils/+bug/2111837; simpler example:

> touch a b
> gnuchown --reference a b
> coreutils chown --reference a b
error: the following required arguments were not provided:
  <FILE>...

Usage: chown [OPTION]... [OWNER][:[GROUP]] FILE...
       chown [OPTION]... --reference=RFILE FILE...

For more information, try '--help'.
> coreutils chown --reference=a b

This likely affects all command-line argument parsing: The GNU argument parser considers --foo X and --foo=X to be equivalent.