sr: session: Keep a global pollfd array

sr: session: Keep a global pollfd array

Currently we keep a file descriptor for each source and construct a pollfd array
from these during each loop iteration in sr_session_run(). This patch modifies
the code to keep a global pollfd array which is only modified when a source is
added or removed. On one hand this gets rid of the constant constructing and
subsequent freeing of the pollfd array in sr_session_run(), on the other hand it
will allow us to implement support for non-fd based pollfds.

Signed-off-by: Lars-Peter Clausen <redacted>