sys/shell/cmds: add cat and tee cmds

[Enoch247]

Contribution description

This patch moves these shell cmds out of the filesystem example into the shell cmds folder. This is done to make them available to all applications.

Testing procedure

• run make -C examples/filesystem all term
• run help
• observe that cat and tee commands are present

Issues/PRs references

none known

[Enoch247]

Since I simply moved these commands, and did not write them, I copied the copyright and author info from the file they originally came from. I checked the git history to verify the author. Is it okay for me to assign a copyright to somebody else?

[Enoch247]

I am wondering if the tee cmd should be renamed to something else. It doesn't really behave much like the *nix tee cmd, which would be a source of confusion for most users. Or maybe just drop it all together?