DBusSocketSet: new abstraction for struct pollfd[] or whatever

DBusSocketSet: new abstraction for struct pollfd[] or whatever

In this second version of this patch, DBusSocketSet is an "abstract base
class" so that when using a better OS-specific API fails, we can always fall
back to _dbus_poll(). For instance, this is necessary when the "better
OS-specific API" is epoll on Linux, the build machine has a modern glibc
version, and the host machine either has an old kernel, is emulated in qemu
(which does not support the epoll syscalls yet), or both.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=33337
Bug-NB: NB#197191
Bug-NB: NB#225019