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

[jhb]

Summary

This typedef is the same a 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.

Test Plan

• booted an amd64 VM in bhyve

[imp]

.Dd bump is the only thing I see...

[kib]

sys/kern/kern_timeout.c

148

The drain function is somewhat strange to convert to the same typedef as callout, but ok.

771

Fix style while there, move local declaration to the start of the function ?

[bcr]

My own callout is this: approved! ;-)

[jhb]

sys/kern/kern_timeout.c

148

While I agree it is strange, the drain function pointer is saved in c_func by callout_drain_async(), and the manpage before my change used timeout_t for the drain function argument to callout_drain_async(). Given those, it seemed reasonable to me to use the typedef for the drain function pointers.