How to support select() in your server

==== Initialize ====

Each select() client is going to need some state, which is defined in
the "struct selclient" from <selfs.h>. Use sc_init on this struct
(usually a part of your per-client state) to set it up.

==== FS_WSTAT support ====

select() support is enabled by the client do a wstat() of certain
entries in the open file. The precise names are not important; what is
important is that the server in its handling of the FS_WSTAT message
calls sc_wstat() (implemented in src/lib/selfs.c in libusr.a) to let
the select support code decode the operation.

If sc_wstat() returns an error, then it didn't recognize the wstat()
fields, and you can go on to check them for your own purposes (or
return an EINVAL if you don't recognize them in your code, either).

If sc_wstat() returns success, then it took care of accepting the
message, so your client has already received a msg_reply() indicating
success. What's left is for you to record that this client is using
select(). Generally, this will entail adding your per-client struct to
a linked list associated with whatever file/object they have open on
this connection.

==== Check select status ====

You will usually have a routine which takes an open file, and then
checks the list of select() users under that file. For any user which
is selecting for something which is available (i.e., they've selected
for reading and there's data ready to be read), a call to sc_event()
is made with a pointer to the "struct selclient" state set up by
sc_wstat(), and also a mask of the event bits active (ACC_READ,
ACC_WRITE, and so forth). sc_event will tell the select server about
this change, and the select server will take care of waking up any
appropriate clients.

This routine should be called whenever a new select() client is added.
It can also be called on event changes on the file (first readable
data arrives, so you need to see if anybody's selecting for readable
data). For the latter, you might find the general routine sub-optimal,
but sufficient.

==== Finish ====

Use sc_done() on any client who's been activated by a successful
sc_wstat(). This will clean up the library's behind-the-scenes state.