MINOR: mux-h2: make h2_wake_some_streams() not depend on the CS flags

MINOR: mux-h2: make h2_wake_some_streams() not depend on the CS flags

It's problematic to have to pass some CS flags to this function because
that forces some h2s state transistions to update them just in time
while some of them are supposed to only be updated during I/O operations.

As a first step this patch transfers the decision to pass CS_FL_ERR_PENDING
from the caller to the leaf function h2s_wake_one_stream(). It is easy
since this is the only flag passed there and it depends on the position of
the stream relative to the last_sid if it was set.