Add a callout_func_t typedef for functions used with callout_*().

This typedef is the same as timeout_t except that it is in the callout
namespace and header.

Use this typedef in various places of the callout implementation that
were either using the raw type or timeout_t.

While here, add <sys/callout.h> to the manpage.

Reviewed by: kib, imp
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D22751