Fix: argpar: Error out when passing an argument to long option that takes no argument

Fix: argpar: Error out when passing an argument to long option that takes no argument

If the user defines a long option `--foo` and passes `--foo=arg`, the
current behavior is that the option foo is recognized and the argument
is ignored.  Change that to be an error.

It is currently not possible to pass arguments to short options using an
equal sign, so this doesn't apply to short options.

Equivalent accepted change in argpar: https://review.lttng.org/c/argpar/+/5361

Change-Id: I6fd6271abb7e271704db1e60f680729931a1fbe1
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>