Create mount points in fs_init() if they don't exist

[aanderse]

In containerized or virtualized environments, standard mount point directories may not exist at boot. Ensure they are created before attempting to mount.

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when running finix in a container we can skip stage 1 (initramfs) entirely - this change let's me boot directly into finit in my stage 2 without the need for a shell script to mkdir and then call finit! 🚀

[troglobit]

I mostly run Finit on systems with read-only root (squashfs), so I'm used to it the hard way, but this is better, much more user-friendly 😃

What do you think about skipping fisdir() and just:

if (mkdir(...) && errno != EEXIST)
    warn("Failed creating mountpoint %s", ...); // strerror(errno) is appended

This may bombard the user with a lot of warnings, but at bringup you want to know, right?