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* Allow an escape hatch for platform specific flags (of which there are many, for example MAP_POPULATE itself is a Linux-only thing, not other Unix). Continue with same defaults as before in this commit, but that really should be changed to *not* include MAP_POPULATE. While pre-faulting all the pages can be useful sometimes *if* you know you're going to access all the data, it is highly unlikely to be what users expect the default to be. For some things all that up front work is 1000s of times slower than being lazy/on-demand/only ever faulting parts of the file. Even the MAP_POPULATE fan who originally in 2014 committed to this file defaulted it to off (but turned it always-on as a "temporary" work around for some long since gone/mutated compiler issue). Anyway, at least something like this `mapFlags` gives users the ability to override the poor default choice or activate any other idiosyncratic platform-specific features. * Use simple, efficient default flags, but also accept whatever the open/mapMem caller specifies. Save flags in MemFile so they can be used in `resize`. This field should not need exporting like the others -- callers can always save whatever values they pass -- but we include a cautionary comment in case anyone ever asks for a `*` there, as well as for documentation. Also make documentation for ``mapFlags`` in open more likely to inspire care.