usr: ls -l, df, mv, tail, tee, sleep, help, poweroff

Filling in the core utilities the box was missing:

- ls -l: type, links and size, now that stat() exists. The screen is 40
  columns, so it is "d  1   256 name", not the GNU sprawl. Sizes were
  previously unknowable except one file at a time with wc -c.
- df: blocks and inodes, used and free, on the 32 KiB cart disk.
- mv: no rename syscall, so it is cp + rm - and the copy has to finish before
  the source goes, or a failure loses the file.
- tail [-n N]: no seek syscall, so it streams the input through a ring of N
  lines rather than jumping to the end. N is capped by that ring.
- tee: the missing half of a pipeline - `cat big | tee copy | wc -l`.
- sleep: the thing every script needs; the kernel counts 1/64 s ticks.
- help: lists every command the shell knows, walked out of the kernel's own
  name table. The box previously had no way to tell you what it could do.
- poweroff: the only safe way to stop a gbos that has data on it. The cart's
  battery RAM is written when the emulator exits cleanly, so killing it loses
  the whole filesystem - it comes back "formatted". Found while testing /rc.

1 files changed, 11 insertions, 0 deletions

diff --git a/usr/poweroff.c b/usr/poweroff.c
new file mode 100644
index 0000000..d57dbc9
--- /dev/null
+++ b/usr/poweroff.c
@@ -0,0 +1,11 @@
+#include "gbos.h"
+/* poweroff: shut the machine down cleanly.
+ * This is the ONLY safe way to stop a gbos with data on the disk: the cart's
+ * battery-backed RAM is only written out when the emulator exits properly,
+ * so pulling the plug (SIGKILL, closing the window) loses the whole
+ * filesystem - it comes back "formatted" on the next boot. */
+void main(void) {
+    puts("poweroff: syncing disk...");
+    nl();
+    poweroff();
+}