sys/vfs: add vfs_unmount_by_path()

[benpicco]

Contribution description

If we mount a file system by path we also might want to unmount it, e.g. for clean shutdown or reboot.

Just adds a function that does the same as vfs_mount_by_path() but with unmount instead of mount.

Testing procedure

Issues/PRs references

Complements #17661

[fjmolinas]

Funny I had the same added in another project 33c02b4, I used it because when using native with gcoap-fileserver I could copy new files in, and re-mounting re-freshed what files native used. I can ACK this right away, unless you would think a rmount function would be useful as well :)

[benpicco]

I also like the vfs mkdir function you added there 😃

The vfs mount vfs unmount commands would sure be useful, want to make a PR for that?

[fjmolinas]

I also like the vfs mkdir function you added there smiley

The vfs mount vfs unmount commands would sure be useful, want to make a PR for that?

Lets get this in then!

[fjmolinas]

ACK!