Enable copy-on-write heap initialization by default (#3825)

* Enable copy-on-write heap initialization by default

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.

* Remove ubiquitous "memfd" terminology

Switch instead to forms of "memory image" or "cow" or some combination
thereof.

* Update new option names

Cargo.toml

@@ -98,14 +98,14 @@ default = [
   "vtune",
   "wasi-nn",
   "pooling-allocator",
-  "memfd",
+  "memory-init-cow",
 ]
 jitdump = ["wasmtime/jitdump"]
 vtune = ["wasmtime/vtune"]
 wasi-crypto = ["wasmtime-wasi-crypto"]
 wasi-nn = ["wasmtime-wasi-nn"]
 uffd = ["wasmtime/uffd"]
-memfd = ["wasmtime/memfd"]
+memory-init-cow = ["wasmtime/memory-init-cow"]
 pooling-allocator = ["wasmtime/pooling-allocator"]
 all-arch = ["wasmtime/all-arch"]
 posix-signals-on-macos = ["wasmtime/posix-signals-on-macos"]