Revert `org-agenda-dim-blocked-tasks' and `org-agenda-inhibit-startup' to their previous default (t and nil)

* org.el (org-agenda-inhibit-startup): Revert to nil as the default.
* org-agenda.el (org-agenda-dim-blocked-tasks): Revert to t as the
default.

The reason for this reversion is that that users expect the agenda to
DTRT by default, and that it's better not to change the previous default
in general.  Also, users who need to speed up their agenda are probably
long time users with big and numerous agenda files, and it's easier for
them to find these variables than it is for newbies to find out why the
agenda does not DTRT.

If users want to speed up their agenda, they can now read advice here:
http://orgmode.org/worg/agenda-optimization.html

lisp/org-agenda.el

@@ -874,7 +874,7 @@ entry, the rest of the entry will not be searched."
   :group 'org-agenda-daily/weekly
   :type 'boolean)
 
-(defcustom org-agenda-dim-blocked-tasks nil
+(defcustom org-agenda-dim-blocked-tasks t
   "Non-nil means dim blocked tasks in the agenda display.
 This causes some overhead during agenda construction, but if you
 have turned on `org-enforce-todo-dependencies',

lisp/org.el

@@ -17309,7 +17309,7 @@ This requires Emacs >= 24.1, build with imagemagick support."
 	  (list :tag "Use #+ATTR* or a number of pixels" (integer))
 	  (const :tag "Use #+ATTR* or don't resize" nil)))
 
-(defcustom org-agenda-inhibit-startup t
+(defcustom org-agenda-inhibit-startup nil
   "Inhibit startup when preparing agenda buffers.
 When this variable is `t' (the default), the initialization of
 the Org agenda buffers is inhibited: e.g. the visibility state