[Issue 21624] New: Typo in `--DRT` or using `help` does not make the program exit

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21624

          Issue ID: 21624
           Summary: Typo in `--DRT` or using `help` does not make the
                    program exit
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: minor
          Priority: P1
         Component: druntime
          Assignee: nobody at puremagic.com
          Reporter: pro.mathias.lang at gmail.com

It is a common expectation for users that using `--help` on a program will
*NOT* start the program. Likewise, when a user makes a typo in the arguments
passed to a program, the program should *NOT* start.

The arguments parsing in DRuntime doesn't do any of this. `--DRT-gcopts=foo`
(notice the extra 's' after `gcopt`) will happily run the program, and so will
`--DRT-gcopt=help`.

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