client: drop unused event queue cond and list variables

The wl_event_queue cond variable has been replaced by the wl_display
reader_cond variable (commit 3c7e8bfbb4745315b7bcbf69fa746c3d6718c305).
This cond variable is never waited for anymore, just
signaled/broadcasted, and thus can be safely removed.

The wl_display event_queue_list and link from wl_event_queue
can be removed as well, since it was only used to iterate over
the event queue list in order to broadcast the now unused cond.

No regression on queue unit tests.

Signed-off-by: Olivier Blin <olivier.blin@softathome.com>

v2: fixed and rebased after 886b09c9a3a9d8672039f09fe7eaf3f2b2b012ca
    added signed-off-by

v3: removed link from wl_event_queue

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
client: broadcast the right pthread_cond variable

In previous commit we removed unused variables. One of them was
pthread_cond_t that was formerly used when reading from display, but
later was (erroneously) made unused. This patch fixes this error
and is a fix for the failing test introduced few patches ago (tests:
test if thread can block on error)

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
client: check for error in wl_display_read_events

This prevents from blocking shown in one display test. Also, it
makes sense to not proceed further in the code of the function
when an error ocurred.

v2. set errno
    put note about the errno into wl_display_prepare_read doc
    check for error with mutex locked

v3.
    set errno to display->last_error

    check for the error only in wl_display_read_events. It's sufficient
    as prevention for the hanging and programmer doesn't need to
    check if wl_display_prepare_read (that was previously covered by
    this patch too) returned an error or the queue just was not empty.
    Without the check, it could result in indefinite looping.

Thanks to Pekka Paalanen <pekka.paalanen@collabora.co.uk> for
constant reviewing and discussing this patch.

Signed-off-by: Marek Chalupa <mchqwerty@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>