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
Merge "Combine SpecialPageBeforeFormDisplay set of hooks into one"