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 ↗

(On Diff #65463)

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

771 ↗

(On Diff #65463)

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 ↗

(On Diff #65463)

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.