Disallow excess arguments

[shadowspawn]

Problem

By default, if is not an error to pass excess command-arguments. This may sometimes be as intended but
will often mean a command-line error by the user will be silently ignored. A nice example from #2149

One particular case is when an argument is a name/title/label, in which the user may include spaces while forgetting quotes.

I originally intended excess arguments to be an error by default, but made it opt-in for the first release to reduce breakage at the time:

• WIP: Reject excess arguments by default #1399 (comment)
• Make new excess arguments error opt-in #1429

See: #2149

Solution

• Make allowExcessArguments false by default.
• Fix lots of broken tests where passing undeclared arguments!
• Add migration tips to CHANGELOG for the breaking change.