Fix shell-dirtrack-mode showing up as enabled in unrelated buffers

shell-dirtrack-mode always shows up in unrelated buffers.

Steps to reproduce:
1. emacs -q
2. M-x shell
3. describe-mode (C-h m) in *scratch* or *GNU Emacs*

This is because shell-dirtrack-mode is aliased to shell-dirtrackp,
which has default value t.

Changes in this patch:
mark shell-dirtrackp obsolete
replace existing occurrences of shell-dirtrackp with shell-dirtrack-mode
add :interactive (shell-mode) suggestion to shell-dirtrack-mode

shell-dirtrack-mode is still turned on by default. There is an
explicit call to (shell-dirtrack-mode 1) in the definition of
shell-mode.

Definition of shell-dirtrack-mode is moved to quiet warning about
reference to free variable ‘shell-dirtrack-mode’.

Note: I am not sure what to put for WHEN on the obsolete variable.
Does that get filled in when merging?

--
Vladimir Sedach