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

include/file.h

@@ -175,6 +175,9 @@ struct inode {
 int iheld(uint dev, uint64 inum);
 // 1 if any inode on dev but `except` is referenced (0: no exception). In fs.c.
 int ibusy(uint dev, uint64 except);
+// Path lookup that follows a final symlink too; 0 with *err set on failure.
+// `start` 0 means the cwd. In fs.c.
+struct inode *nameifollow(struct inode *start, char *path, int *err);
 
 // table mapping major device number to
 // device functions