modem: Rework the modem_sockets poll behavior

The current modem sockets poll implementation has 2 limitations
as of today:
- not following posix spec wrt timeout of -1 (should be forever,
  but as today it's was returning immediately)
- not able to poll from multiple threads on different sockets
  on the same modem.
This pull request should implement these limitations.

Signed-off-by: Wouter Cappelle <wouter.cappelle@crodeon.com>