Fix signals and offline mode for js/wasm

[hslatman]

Changes in this PR include the things I needed to change in the CLI to make it compile to js/wasm.

Had to move around and refactor some code to fix import cycles and such. I'm not completely happy with the structure yet, but it's a start.

The js/wasm build is not fully functional. There are things that are not supported in a browser environment, so these functions can't be used. Using mTLS (easily) is one of them. There are also things that aren't fully implemented yet, like the --offline mode. Currently this functionality is stubbed, so that the js/wasm build can at least compile. It would be cool to support this in the future, though, as this would allow running an offline instance of step-ca in memory in a browser.

Running GOOS=js GOARCH=wasm go build -o ./html/test.wasm -tags nobadger,nobbolt cmd/step/main.go will not result in a Wasm module yet. There are still some changes in dependencies to be completed:

# github.com/chzyer/readline
vendor/github.com/chzyer/readline/utils.go:81:29: undefined: State
vendor/github.com/chzyer/readline/utils.go:241:9: undefined: State
vendor/github.com/chzyer/readline/operation.go:234:4: undefined: ClearScreen
vendor/github.com/chzyer/readline/readline.go:129:20: undefined: GetScreenWidth
vendor/github.com/chzyer/readline/readline.go:132:22: undefined: DefaultIsTerminal
vendor/github.com/chzyer/readline/readline.go:142:26: undefined: DefaultOnWidthChanged
vendor/github.com/chzyer/readline/remote.go:324:2: undefined: DefaultOnWidthChanged
vendor/github.com/chzyer/readline/remote.go:346:17: undefined: GetScreenWidth
vendor/github.com/chzyer/readline/remote.go:362:16: undefined: DefaultIsTerminal
vendor/github.com/chzyer/readline/terminal.go:51:2: undefined: SuspendMe
vendor/github.com/chzyer/readline/terminal.go:51:2: too many errors
# github.com/newrelic/go-agent/internal/sysinfo
vendor/github.com/newrelic/go-agent/internal/sysinfo/usage_posix.go:17:17: undefined: syscall.Getrusage
vendor/github.com/newrelic/go-agent/internal/sysinfo/usage_posix.go:17:35: undefined: syscall.RUSAGE_SELF
# github.com/smallstep/zcrypto/x509
vendor/github.com/smallstep/zcrypto/x509/cert_pool.go:38:9: undefined: loadSystemRoots
vendor/github.com/smallstep/zcrypto/x509/root.go:21:32: undefined: loadSystemRoots

In my POC repo, using forks of those with dirty hacks, the Wasm compiles with the changes from this PR.

[hslatman]

There's a (recent) open PR for js/wasm target for newrelic/go-agent: newrelic/go-agent#478. That should work for us too. It's an indirect dependency through github.com/smallstep/certificates/monitoring.

We're already maintaining our own fork of github.com/smallstep/zcrypto/x509, but it's heavily outdated and it has changed a lot internally. We should be good for now to just make this build with a small patch. We can look into getting closer to upstream later. My PR to the upstream for js/wasm support was already merged: zmap/zcrypto#335.

The hardest to get working correctly is github.com/chzyer/readline. It's possible to stub out the required functions, and that's what I did in my POC, but I don't think that's acceptable to the upstream.