Combine SpecialPageBeforeFormDisplay set of hooks into one

Combine SpecialPageBeforeFormDisplay set of hooks into one

Hooks with dynamically generated names cannot be documented in the normal
manner, and indeed this set, added in r86482 / 10d93c34cb17, was not.

In contrast, every other hook in core has a name that is a fixed string
and thus is easy to grep for, create a manual page title for, and so on.
Conceptually, each hook's name should correspond to one or more locations
at which code can be inserted.

So I changed the code to use hook name "SpecialPageBeforeFormDisplay"
for all FormSpecialPages and added the special page name as a parameter,
which is consistent with existing hook "ActionBeforeFormDisplay".

From extensions in Gerrit, the only use I found was in StopForumSpam,
which is updated to use the new hook in Id474915a. So I chose to break
backward compatibility instead of using a hack to hide the old hooks
from maintenance/findHooks.php.

After three years, the script again reports, "Looks good!"

Change-Id: I7ea6521b47fb034bc367a1d06d477a7654035a5f