kernel+init: migrate /dev /proc /tmp mounts to userspace init

kernel+init: migrate /dev /proc /tmp mounts to userspace init

Add devfs, procfs, and auto-mkdir support to SYSCALL_MOUNT so that
userspace can mount virtual filesystems via the mount() syscall.

Userspace migration:
- init (user/cmds/init/init.c) now calls mount() directly to mount
  devfs on /dev, procfs on /proc, and tmpfs on /tmp before spawning
  the shell or running inittab entries.
- This mirrors how Linux init handles these mounts from userspace.

Kernel fallback:
- Kernel-side mounts for /dev, /proc, /tmp are kept as fallback for
  non-init boot paths (e.g. init=/sbin/fulltest). When /sbin/init
  runs, it re-mounts these; vfs_mount replaces existing entries so
  this is a harmless overlap.

Other changes:
- SYSCALL_MOUNT: add devfs and procfs as recognized fstype strings,
  with auto-mkdir of the mountpoint directory (like Linux mount(8))
- Makefile: add /etc/init.d/rcS to initrd (placeholder for future
  shell-script-based startup when shebang/-c support is added)
- rootfs/etc/init.d/rcS: placeholder script