kernel - Fix /dev/fd/N and clean up the old dup error-code-driven path

kernel - Fix /dev/fd/N and clean up the old dup error-code-driven path

* When opening /dev/fd/N, replicate the file pointer for descriptors
  that represent vnodes instead of dup()ing.  This ensures that the seek
  offset and other fp-related elements are not shared unexpectedly.

* Refactor the open() path to allow dev_dopen() to replace the
  struct file by passing a struct file ** instead of a struct file *.
  This removes old error-code-based hacks.

* This fixes the shared seek position that fexecve() was operating with
  due to its use of /dev/fd/N for scripts.

Reported-by: aly