Adjust "context_menu_disabled" configuration.

Adjust "context_menu_disabled" configuration.

This @fix three (yes 3!) errors from commit cafda5928f9bf8116564219dc03d12c3a6e131b3

1. the initial value in the standard config is wrong, causing ctx menu to be disabled for everyone!
2. the argument of the api should be named "disabled" (like all the other disable API in this context)
3. (minor) fixed doc for elm_config_context_menu_disabled_set

NOTE: there are no configuration gui for this option, if you cleared your elm config recently you probably now have the ctx menu disabled everywhere, only solution is to wipe your config (to let elm create a new correct one)

Big SPANK to the author, the commiter and all the reviewers of D2463!!!

Change-Id: I92ddb0659d64d8dd55150aa22e7d8ab3a61bc43c