select: Externalize fd_set filling and dispatch

To integrate with an external event loop (in this case glib) we
need to allow an application to get a filled out fd_set and then
dispatch it. osmo_fds and maxfds are static and I decided to keep
it that way and instead create two routines to fill the fdset and
then one to dispatch the result.

The public header file does not include sys/select.h and we can
compile the library without select, so I didn't want to require
having to include this file, and used void * for the parameter.

Mark the routines as inline to avoid a call from the select
function. I have confirmed that inlining has an effect on x86
using Debian's gcc-4.9.2-10 compiler
2 files changed