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

crates/runtime/Cargo.toml

@@ -47,6 +47,7 @@ maintenance = { status = "actively-developed" }
 
 [features]
 default = []
+memory-init-cow = ['memfd']
 
 async = ["wasmtime-fiber"]