MEDIUM: twork: Introduce a new cross-thread/thread-group deferred work API

MEDIUM: twork: Introduce a new cross-thread/thread-group deferred work API

Introduce twork, a way to execute code on another thread, or another
thread group. The main motivation for that was to be able to obtain
information about file descriptor belonging to other thread groups, when
running with per-thread-group file descriptor tables.
The functions are :
int twork_call_on(struct twork *tw, int tid,
                  union twork_arg (*fn)(union twork_arg),
                  union twork_arg arg,
                  void (*done)(union twork_arg arg, union twork_arg ret));
Runs function fn on thread tid, providing arg as an argument, and
calling done from the caller thread when the work is complete.
int twork_call_tgroup(struct twork *tw, uint tgid,
                      union twork_arg (*fn)(union twork_arg),
                      union twork_arg arg,
                      void (*done)(union twork_arg arg, union twork_arg ret));
Same thing, but running on thread group tgid instead.

"done" is optional, if there is no need to be notified that the work is
complete.
"tw" is optional, if none is provided, one will be allocated
dynamically. Note that you can only cancel a twork if you're using one
that you provided by yourself.

int twork_cancel(struct twork *tw);
Cancels the tw, assuming it is not already running. It will return 1 on
success, and 0 on failure, in which case the work will be completed, and
the "done" function will be called.