Use the new command-handler initializer macros in dexter;

Use the new command-handler initializer macros in dexter;
clean up the resulting warnings.

Fix minor glitch in previous changes to mpmt_pthread: the const-ness
of ap_scoreboard_fname wasn't consistent between mpmt_pthread.c and
scoreboard.c (of course, putting the decl in a header file would have
helped root this out earlier).

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@85614 13f79535-47bb-0310-9956-ffa450edef68