kernel: chroot, #! scripts, rename and mmap as apk needs them

Found by running Alpine's apk on SchOS, one failure at a time:

- chroot(2) was a stub returning EPERM. Each process now has a root:
  where an absolute path starts and where .. stops. Mounts remember
  whose root they were made under, and a walk leaves a mount by the way
  it came in, so one filesystem can be mounted twice.
- devtmpfs and proc mounted only on /dev and /proc, whatever the target;
  a chroot got "already mounted".
- mmap never reused freed address space, so musl's malloc used up the
  256 mapping records while 780 MB were free. A mergeable mapping now
  goes straight after an anonymous one whose end has room.
- unlinkat returned -1 (EPERM) where apk expects ENOENT, and renameat
  was an old copy of rename: no replacing the target, and v1 dirents
  written into SchFS. Both now share rename's implementation.
- execve runs #! scripts, and says ENOEXEC for what it cannot run.

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

kernel/fs/devfs.c

@@ -318,18 +318,19 @@ const struct fs_ops devfs_ops = {
   .getdents  = devfs_getdents,
 };
 
-// Graft it over /dev. Called by init, which has made the directory.
-void
-devfs_mount(void)
+// Graft it over `target` -- /dev from init, and again wherever a chroot
+// wants one. It was always /dev whatever was asked for, so a second mount
+// failed as "already mounted" and a chroot had no devices.
+int
+devfs_mount(const char *target)
 {
-  struct inode *dp = namei("/dev");
+  struct inode *dp = namei((char *)target);
+  int r;
 
-  if (dp == 0) {
-    cprintf("devfs: /dev does not exist; not mounted\n");
-    return;
-  }
-  if (mount_add(dp->dev, dp->inum, DEVFS_DEV, DEVFS_ROOT, "/dev", "dev",
-                "devtmpfs") < 0)
-    cprintf("devfs: /dev already has something mounted on it\n");
+  if (dp == 0)
+    return -ENOENT;
+  r = mount_add(dp->dev, dp->inum, DEVFS_DEV, DEVFS_ROOT, (char *)target, "dev",
+                "devtmpfs");
   iput(dp);
+  return r < 0 ? -EBUSY : 0;
 }