proc: Keep /proc/self cached separately from /proc/pid

When dumping tasks we do a lot of open_proc()-s and to
speed this up the /proc/pid directory is opened first
and the fd is kept cached. So next open_proc()-s do just
openat(cached_fd, name).

The thing is that we sometimes call open_proc(PROC_SELF)
in between and proc helpers cache the /proc/self too. As
the result we have a bunch of

  open(/proc/pid)
  close()
  open(/proc/self)
  close()

see-saw-s in the middle of dumping tasks.

To fix this we may cache the /proc/self separately from
the /proc/pid descriptor. This eliminates quite a lot
of pointless open-s and close-s.

Signed-off-by: Pavel Emelyanov <xemul@parallels.com>