To keep the system module focused on the System trait, this commit
extracts the SelectSystem struct and related items to a new file.

This commit updates the `SharedSystem` methods `set_nonblocking`,
`reset_nonblocking`, `read_async`, `write_all`, and `print_error` to
take immutable references (`&self`) instead of mutable references
(`&mut self`).

This change is made to reflect the fact that these methods do not
mutate the `SharedSystem` instance itself, but rather the `SelectSystem`
instance that it contains.

Previously, when you have a non-mutable reference to a `SharedSystem`,
you had to clone it to call these methods. Now, you can call them
directly.