* Add an entrypoint for calling open that bypasses the varargs. * Add an entrypoint for calling openat that bypasses the varargs.
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Add at least a one-line comment on every `#ifdef __wasilibc_unmodified_upstream` briefly indicating what its purpose is. While here, reenable some of the code that was previously disabled when getenv wasn't supported, as getenv is now supported.
This incoporates pieces from musl-libc, cloudlibc, cloudabi, libpreopen, and dlmalloc, as well as a significant amount of new code.
