[RFC] Implement listening on activation environment changes

[q66]

This implements a protocol that lets a connection to the control socket listen on changes in the global environment issued by dinitctl setenv. This opens up a range of possibilities - such as special services that trigger other services when an environment variable appears.

It also implements a dinit-monitor environment watching mode as a proof of functionality, though likely not acceptable as is (likely at least the string_view will need getting rid of?)

I intentionally did not implement things like protocol checking and whatnot yet since I want your feedback first.

I should probably also change the protocol a bit - at very least, the envevent should contain information about whether the envvar is newly created or if it overwrites a previous one.

Let me know what you think :)

[q66]

i think the code quality in this is ok now, what's missing from my side is imo:

1. documentation
2. tests
3. the protocol bit about new/overriden envvars

[q66]

i also added an option to dinit-monitor that makes it exit upon first issued command; that means you can invoke something like

$ dinit-monitor --initial --env --exit --command /usr/bin/true FOO BAR BAZ

as a way to wait until either of the given environment variables becomes available (with --initial it should be race-free and will simply exit when it's already available)

[q66]

There is another thing I would like to have while at it and that is unsetting of environment vars. As I see it, we don't have to add a protocol message for it; we can repurpose the SETENV message and instead of raising BADREQ when the value has no =, we could just change

    eq = envVar.find('=');
    if (!eq || eq == envVar.npos) {
        // Not found or at the beginning of the string
        goto badreq;
    }

    main_env.set_var(std::move(envVar));

to

    eq = envVar.find('=');
    if (eq == envVar.npos) {
        // Unset
        main_env.undefine_var(std::move(envVar));
    } else if (!eq) {
        // At the beginning of the string
        goto badreq;
    } else {
        main_env.set_var(std::move(envVar));
    }

and add a new command to dinitctl that will invoke the protocol in this form...

thoughts?