fs: inode and open-file tables that grow, with a hash for iget

The inode table was NINODE (200) entries, searched end to end on every
iget(), and a panic -- "iget: no inodes" -- when all of them were held.
Held means open, a cwd, a running binary or a directory mid-walk, so a
few busy processes reached it. iget() now finds an inode through a hash
on (dev, inum), takes a free one from a list, and makes more from a
slab cache once the NINODE in the image are all in use.

The open-file table had the same shape, NFILE (512) and then ENFILE,
and gets the same treatment: a free list, and the slab past NFILE.
/proc/fsstat's peaks now read against how many exist.

getrlimit(RLIMIT_NOFILE) said unlimited, which musl turns into
sysconf(_SC_OPEN_MAX) == -1, after which zsh believed it had no
descriptors and `exec {fd}<file` failed on the first one. It reports
NOFILE now. And a comment in sys_fs.c held a "*/" that ended it early.

Checked under QEMU: six processes holding 115 files each, 713 open
descriptions and 341 inodes at once, all back to the baseline after.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S1giNvrHTrPMPw7V6x1kQ4

kernel/sys/sys_fs.c

@@ -16,7 +16,7 @@
 #include "errno.h"
 #include "schfs.h"      /* SCHFS_BLKSIZE, SCHFS_NAME_MAX — statfs reports them */
 #include "gfx.h"        /* gfx_cols/gfx_rows — TIOCGWINSZ reports them */
-#include "vt.h"         /* the terminals TIOC*/VT_* act on */
+#include "vt.h"         /* the terminals the TIOC and VT_ ioctls act on */
 #include "pciuser.h"
 #include "keymap.h"    /* KB_*LAYOUT — the keyboard layout catalogue */
 #include "netuser.h"