rts: Fix --debug-numa mode under Docker

As noted in #17606 (closed), Docker disallows the get_mempolicy syscall by default. This caused numerous tests to fail under CI in the debug_numa way. Avoid this by disabling the NUMA probing logic when --debug-numa is in use, instead setting n_numa_nodes in RtsFlags.c.

Fixes #17606 (closed).

Also makes the RTS command-line parser more strict about the form of the --numa flag (an issue which had complicated the debugging of this patch).