ctl: Make ctl.h more self-contained

Make MALLOC_DECLARE conditional on <sys/malloc.h> and forward declare
several types.

Reviewed by: ken, imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D42206

(cherry picked from commit 55231cd180692c7299bff65e608dba20013ec0f7)