fat: FAT12, FAT16 and FAT32 with long names, read and write

The filesystem every other system can read: a USB stick, an SD card, the
EFI system partition. It is here first for the hardware report, which has
to leave a machine on something that can be read on another one.

  mount -t vfat /dev/usb0p1 /mnt        (msdos and fat work too; -o ro)

Written from Microsoft's specification, and matched to Linux vfat where
the specification leaves a choice: 8.3 names with the NT lowercase bits,
long-name entries otherwise, Linux's alias rule (REPORTS, then BIGNUM~1),
case-insensitive lookup, UTC times, the boot-sector dirty bit, every FAT
copy and FSInfo kept up to date. It uses its own sector cache over the
new blkdev_xfer, since a partition need not be a whole number of 4 KB
blocks, and every operation that changes something reaches the disk
before it returns. Inode numbers come from the entry position, or the
first cluster for a directory, plus a small table for entries that moved.

Around it:
- /dev lists every disk and partition the block layer found (major 8,
  minor = block device), and raw reads and writes on them go by sector.
- The ESP and MBR FAT partitions are registered, typed so the root probe
  passes over them. MBR entries with a bad status byte are skipped, so a
  superfloppy's boot code is not read as a partition table.
- statfs, sync, fsync, umount (EBUSY while in use), shutdown, reboot and
  the power button all know about FAT.
- create undoes and reports a failed ialloc or dirlink (ENOSPC, EINVAL,
  EPERM) instead of panicking; fs_ops gains refuse_types and no_hardlinks.
- filewrite returns a short write instead of panicking on it.
- utimensat is implemented. As a stub returning 0 it made toybox touch
  believe the file existed, so `touch newfile` created nothing on any
  filesystem.

Tested in QEMU against mkfs.vfat images (FAT12 floppy, FAT16 on MBR,
FAT32 superfloppy and GPT ESP) by writing, renaming, deleting, filling to
ENOSPC and truncating there, then checking with fsck.fat -n and mtools on
the host: all clean, contents match.

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

kernel/sys/sys_proc.c

@@ -10,6 +10,7 @@
 #include "uaccess.h"
 #include "errno.h"
 #include "acpi.h"
+#include "fat.h"
 
 addr_t sys_fork(void)      { return fork(); }
 addr_t sys_wait(void)      { return wait(); }
@@ -153,6 +154,9 @@ sys_uptime(void)
 addr_t sys_reboot(void)
 {
     cprintf("System is rebooting...\n");
+    // A stick left mounted should come back readable, and marked clean.
+    schfs_ksync_all();
+    fat_ksync_all(1);
     // The FADT's reset register first: it is the one the board's designer
     // wired up. Then the PCI reset port and the keyboard controller, which
     // are what nearly every PC has had since there were PCs.
@@ -170,6 +174,7 @@ addr_t sys_shutdown(void)
     // kept for a machine whose AML did not load; a real board answers none
     // of them.
     schfs_ksync_all();
+    fat_ksync_all(1);
     acpi_poweroff();
     outw(0x604, 0x2000);
     outw(0xB004, 0x2000);