Raise number of threads used by GASNet by default + add warning if exceeded

[bradcray]

As noted in #26379, we've recently had a user on a high-core-count system get surprised when we quietly dialed the maximum number of tasks down to 255 rather than using the number of cores on that system. This PR addresses that issue in two ways:

• by inserting a warning if/when this happens so that the user can be aware that it is happening (it's admittedly not a very good warning, but it's also now even less likely that it will occur, because we also…)
• by raising the number of threads that GASNet is configured with to 65535 as suggested by @bonachea in GASNet's max threads per process silently limiting Chapel's maxTaskPar #26379 (comment), which will result in 1024 being used as the default limit

While here, I also noticed that we seemed to be passing the list of GASNet configuration options to it twice, introduced in 2014 in d765982. I don't see any indication that that's any more than a typo, so here, I've removed one of the occurrences, leaving the one at the end of the line there, thinking it'd support overriding other, earlier options.

[bonachea]

Thanks for the fixes!

Made a few minor observations, in case they're helpful.

[bonachea]

New warning text LGTM, and should usually achieve the goal of avoiding silent under-utilization of cores.

However IIUC the new warning won't be printed on a heterogeneous system where node 0 has a smaller core count than maxThreads, but some other node might have a higher core count.

Not sure if such node heterogeneity is of interest (or even permitted by Chapel?) so feel free to ignore this observation if it seems irrelevant or not worth addressing.

[bonachea]

This hunk partially overlaps the same fix proposed in the first commit of #25414 for the same cosmetic defect.

It sounds like we both independently made the same observation about the GASNet configure line and arrived at the same fix, so feel free to close #25414 in favor of this PR.