Revert `org-agenda-dim-blocked-tasks' and `org-agenda-inhibit-startup' to their previ...

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