fix warnings and stdio flushing

Cargo.lock

crates/arch-x86_64/src/lib.rs

crates/kernel/src/syscall.rs

crates/kshell/src/cmd.rs

crates/kshell/src/shell.rs

crates/libstd/src/io/mod.rs

crates/mem/Cargo.toml

crates/slab-alloc/src/slab.rs