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

include/core/proc.h

@@ -313,6 +313,10 @@ struct proc {
   // rt_sigreturn. See "The FPU" above scheduler() in kernel/proc/proc.c.
   uint8  fpu[512]    __attribute__((aligned(16)));
   uint8  sigfpu[512] __attribute__((aligned(16)));
+
+  // chroot(2): where an absolute path starts, and where ".." stops. 0 is the
+  // system's root. Inherited across fork, held as a reference like cwd.
+  struct inode *root;
 };
 #define FD_CLOEXEC 1