uffd: fix eager heap init bug, add testing, update defaults to reflect production

[pchickey]

• bugfix in the eager heap initialization: guard against initializing zero-sized memory. the zeropage ioctl will helpfully return EINVAL when this code path is exercised with initial_size = 0, e.g. when your wasm has (memory (export "memory") 0)
• add better testing coverage, checking the existing uffd test with both lazy (default, as before) and eager heap disposition, and add a test specifically for the bug fixed above.
• update impl Default for UffdConfig to the values we use in production.

[acfoltzer]

• add better testing coverage, checking the existing uffd test with both lazy (default, as before) and eager heap disposition, and add a test specifically for the bug fixed above.
• update impl Default for UffdConfig to the values we use in production.

Are there commits missing here?

[pchickey]

yep sorry I pushed to the wrong remote.