Align 'es_poll' semantics closer with POSIX.

* src/estream.c (_gpgrt_poll): Do not return early if some streams are
found to be ready before even polling the others.

POSIX mandates that all streams are examined and returning early
violates that. It also imposes an order on how the callee serves the
streams, which might be incompatible with the order the callee wants
to serve them, leading to deadlocks.

• Signed-off-by: Justus Winter <justus@g10code.com>