callout: ddb: resolve symbol of callout function

[bz]

Summary

In the ddb show callout function try to resolve the symbol of the
callout function to improve debugging. In my case I went through
various callouts from show ktr to check what they were and this saved
me opening lldb/gdb next to it (and still having the old kernel as
the panic to debug was upon reboot).

Sponsored by: The FreeBSD Foundation
MFC after: 3 days

[rlibby]

Looks good. I presume you've tested it?

Just noting two points of consistency, though neither really matters:

• You cast to db_addr_t, but the type of db_printsym off is db_expr_t, and _show_last_callout casts to db_expr_t. However I note that all other casts for db_printsym besides _show_last_callout cast to db_expr_t.
• _show_last_callout doesn't add the parens like you do now for _show_callout. I prefer the parens.