feat(userspace): shell runs programs (ls, cat) via spawn w/ terminal+storage+args page; ext4 holes read as zeros; storage re-attach same buffer

docs/ipc.md

@@ -74,7 +74,10 @@ calling it (`types::log`) if init gave it a log endpoint.
 
 A thread made by `SpawnThread`/`SpawnThreadIn` starts with the caller's
 `arg` in `a0`. A process's first thread reads it as `types::start_arg`:
-the slots of the console and log capabilities it was handed.
+the slots of the console, log and storage capabilities it was handed, and
+whether its parent put an arguments page at `ARGS_VA` (NUL-separated, the
+program's name first; the parent fills a page and `MapPage`s it over, so
+the program owns it and it goes when the program does).
 
 Boot loads only init and storage, both packed into the kernel image (they
 are what reads the disk, so they cannot come from it). The disk is one ext4
@@ -102,7 +105,12 @@ font. Its output side is the log protocol (`types::log`), so a program whose
 log endpoint is the terminal prints there with `println!`: standard output
 is whatever text endpoint a program was given. `READ` (`types::terminal`)
 blocks until a line is typed and returns it in chunks, the last ending in
-`\n`. The desktop is the shell, given the terminal.
+`\n`. The desktop is the shell, given the terminal, storage and a spawn
+allowance. A command that is not a built-in runs `name.elf`: the shell
+spawns it, gives it call-only copies of the terminal (as its log, so its
+output) and of storage, writes its arguments page, starts it and waits for
+its death. The programs share the shell's storage badge; storage hands a
+client that attaches again the same buffer, so they run one at a time.
 
 Only init holds `ProcessCreate`. Everyone else runs programs through init's
 spawn service (`types::spawn`, client `libkos::spawn`), which init serves