net: consolidate fake/simulated stuff for JS and NaCl

[mikioh]

This is a proposal to merge the fake network functionality on JS and the simulated network functionality on NaCl into new built-in simulated network functionality in the package net for the sake of maintenance.

Problem: The fake stuff on JS is incomplete, causes #19229 and similar issues and disables most of the test cases in the package net. It also introduced unnecessary dependencies on the packages in the standard library as "stuff making compiler happy."

Solution: To make new simulated network functionality based on NaCl's. The new functionality in a single code is able to run on both JS and NaCl without functional degradation from the existing NaCl's.

Bonus: Probably we are able to use host-local IPv6 functionality on the play.golang.org.

[rsc]

It seems fine to merge the two - just don't expose any new public API in the process.
Is it just a matter of renaming *_nacl.go to *_fake.go and adding build tags?

[mikioh]

just don't expose any new public API in the process.

No new APIs, some deprecated APIs in the package syscall.

Is it just a matter of renaming *_nacl.go to *_fake.go and adding build tags?

Some more. The CL, http://golang.org/cl/120958, chops syscall/net_nacl.go into net/fd_fake.go and net/ipsock_fake.go, fills missing/broken features required for testing, and a few cleanups.

[bradfitz]

Closing nacl bugs, as we're removing nacl (#30439)

If all that's left is js/wasm here, there's nothing to unify. (See https://go-review.googlesource.com/c/go/+/199499 which renames some *_nacljs.go files to *_js.go)