Added unix-like shell/cmd with some commands

[Amjad50]

Fixes #10

This is not just for shell, but general features and fixes were added, including:

• better handling of files/pipes/devices, etc... in the kernel and also in Rust (added RawFd and other neat stuff similar to unix)
• Added file blocking mode options controllable from user mode for all types of files
• added wait_for_pid with an option to not block (i.e. not wait)
• Added a way (implemented in init) to monitor key presses and act as a real terminal

Added unix-like shell with small programs

Now we have:

• echo
• cat