fs: follow symlinks like Linux; Linux mknod; /proc/partitions

Symlinks are followed in namex_from() now, in the middle of a path as
well as at the end, and a relative target resolves from the directory
that holds the link rather than from the caller's cwd. `ln -s f d1/s;
cat d1/s` said "Symbolic link loop" because it looked for ./f. A loop is
ELOOP after 40 links, a dangling link ENOENT, and open(O_CREAT) through
a link to a file opens the file. divesymlink() is gone.

mknod from a Linux program passes (mode, dev) where the native call
passes (major, minor), and was read as the native one: `mknod n c 1 2`
made node 432,258, and mkfifo made a character device. Both are decoded
now; a FIFO is EPERM, and an existing name is EEXIST.

/proc/mounts listed devtmpfs twice: once from a fixed line and once
from the mount table. The fixed line is gone, and mount_add() refuses
a filesystem mounted on its own root, which is where a second devtmpfs
mount lands. A read-only FAT mount says ro.

/proc/partitions exists, in Linux's format, and /proc/filesystems lists
vfat.

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

kernel/fs/procfs.c

@@ -103,6 +103,8 @@ static const struct global_file globals[] = {
   { "bootdebug",   30 },
   // What the slab allocator is holding. See include/kmem.h.
   { "slab",        20 },
+  // Disks and partitions, in Linux's format. See blkdev_info().
+  { "partitions",  21 },
   { "kmsg",        17 },
 };
 #define NGLOBALS ((int)(sizeof(globals) / sizeof(globals[0])))