tang-gen: endless loop if dbdir doesn't exist or no write access

[frasertweedale]

This do-while in tang-gen.c is problematic. I'm not sure why loop is needed but
either existance/access checks are needed or, as a last resort, bail out after
a set number of loops.

Happy to patch this myself but needed to find out why the do-while is there
first :)

} else {
    do {
        r = keyfilegen(dbdir, filename);
        if (r != 0)
            error(EXIT_FAILURE, r, "Error generating keyfile name");

        file = fopen(filename, "wx");
    } while (!file);
}   

[npmccallum]

The loop is there to catch random name collisions (unlikely, but a possibility). keyfilegen() creates a random filename and fopen(filename, "wx") creates the file with that name. If it fails, it may indicate a name collision. Maybe something like:
while (!file && errno == EEXIST)