kernel - Fix kern.proc.pathname sysctl

kernel - Fix kern.proc.pathname sysctl

* kern.proc.pathname is a sysctl used by programs to find the path
  of the running program.  This sysctl was created before we stored
  sufficient information in the proc structure to construct the
  correct path when multiple aliases are present (due to e.g. null-mounts)
  to the same file.

* We do have this information, in p->p_textnch, so change the sysctl to
  use it.  The sysctl will now return the actual full path in the context
  of whomever ran the program, so it should properly take into account
  chroots and such.