The shell runs /bin/<name> by asking init to load it (init alone may
create processes), then gives the program the terminal as its output
and a read-only storage grant to the current directory, writes its
arguments onto a page it moves into the program, starts it and waits
for it to die. Then it takes the grant back. Ctrl+C kills it.

A program that needs more says so in /bin/<name>.manifest, e.g.
`needs = display` for snake, which then gets the screen and keyboard
until it quits. A need the shell does not know, it refuses to run.

Built-ins: help, echo, clear, cd, pwd.