writers: Unify ->pre_select().

writers:  Unify ->pre_select().

Always treat the easy cases "nothing to do", "error", and "not yet initialized"
first. For the alsa writer,  this change fixes two minor bugs:

First, if data is available but alsa has not yet been initialized, we return
from ->pre_select() without setting a delay. This is wrong, we should init
the alsa handle ASAP in this case.

Second,  on errors we wait 20ms which is both ugly and unnecessary.
Requesting a minimal delay is the right thing to do here as well.