stdlib: Version of readback that returns error code when the system command fails

[rsaarelm]

Possible implementation:

(int | string) readbackErr(string command) {
    import std.file : readText, remove;
    import std.string : toStringz;

    auto tmp = tmpnam(null).cToString;
    // XXX: Pipe stderr to devnull to make program errors not propagate to our stderr.
    int iret = neat_runtime_system_iret(command ~ " > " ~ tmp ~ " 2> /dev/null");
    auto ret = readText(tmp);
    remove(tmp);
    if (iret)
        return iret;
    return ret;
}

[FeepingCreature]

Should probably be implemented as a whole Process class with stdin/stdout/stderr redirection options and a .status flag, so we don't need fifty functions for fifty combinations.