sockets/posix: poll()/select() implementation should be factored out from sockets subsys into POSIX subsys

[pfalcon]

POSIX poll() was initially implemented in the scope of BSD Sockets API, and currently located in subsys/net/lib/sockets/ . It is however a generic POSIX facility, and was generalized to apply to arbitrary file descriptors (and objects represented by them). As we grow implementations of such objects (eventfd, unix domain sockets), dependency on networking sockets becomes a problem, as requires enabling unrelated config options and leads to pulling more code into user applications.

To address this, poll() (and by extension, select()) implementation should be migrated to lib/posix/.