shell / ioManager: don't allow backspace when input buffer is empty

fix issue with printing where the first message would not get printed

move 'gets'-functionality to ioManager to simplify the shell source code

add await syscall

check if a message should be printed to the main vga screen by checking if any syscall in the 'syscall stack' is the foreground process

pass keyboard input to shell

remove old log function, make \n mandatory for printing a new line

route the shell service to the vga output

rewrite general IO using an IO-manager