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/exec.c

@@ -11,6 +11,10 @@
 #include "errno.h"
 #include "x86.h"
 #include "fsstat.h"
+#include "spinlock.h"
+#include "sleeplock.h"
+#include "fs.h"
+#include "file.h"
 
 // Binaries with entry points at 0x100000+ are musl/Linux-ABI.
 // SchOS-native binaries (init) are linked at 0x4000.
@@ -56,11 +60,12 @@ exec(char *path, char **argv, char **envp)
 
   begin_op();
 
-  if((ip = namei(path)) == 0){
+  int err;
+  if((ip = nameifollow(0, path, &err)) == 0){
     end_op();
     // Must be ENOENT, not a bare -1: execvp() relies on this exact errno to
     // know it should try the next $PATH component rather than give up.
-    return -ENOENT;
+    return -err;
   }
 
   ilock(ip);