Fixes#515
This includes a few small and closely related changes:
1. All occurrences of the build tag `+build js` have been changed to the
more precise `+build js,wasm`. This will exclude the files from being
included by third-party compilers like GopherJS, with which they are
incompatible.
2. Some files which are incompatible with JavaScript/Wasm now have the
correct build tag (`+build -js`) so they will be excluded from Wasm
builds.
3. Some configuration options which are incompatible with
JavaScript/Wasm (or at least the current bindings) will now no longer
appear in Wasm builds. This meant creating new files with new struct
definitions and the appropriate build tags.
