Enable copy-on-write heap initialization by default

[alexcrichton]

This commit enables the Config::memfd feature by default now that it's
been fuzzed for a few weeks on oss-fuzz, and will continue to be fuzzed
leading up to the next release of Wasmtime in early March. The
documentation of the Config option has been updated as well as adding
a CLI flag to disable the feature.

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[cfallin]

Thanks!

A random thought occurs: now that this feature has been generalized to work with generic Unix and mmap files straight from disk, with no actual memfd_create(), I wonder if there's a better name for it, like "copy-on-write heap images"? (cow_heaps?) To be clear I don't think it absolutely needs to change; it's well-documented exactly what it means in any case; but it might avoid some confusion at the margins in the future if it doesn't refer to the Linux-specific feature.

[cfallin]

maybe we can put "Unix" after "Linux" in this list and say "Other Unix" instead, for clarity?

[alexcrichton]

Seems reasonable to me! I've switched the config option to memory_init_cow and also ran a pass over the internals to rename "memfd" to something along the lines of "memory image" or "cow" or something like that.

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[alexcrichton]

I went ahead and merged #3831 because I read that first and rebased here to update the names/docs w.r.t. "memfd" terminology