Set exception on channel in parking thread when POLLPRI event detected.

Set exception on channel in parking thread when POLLPRI event detected.

This is done just to make the code be equivalent to the old select code. As
noted in 303106 the same issue was already fixed in this branch, but the
exception was not set on the channel in the case of POLLPRI. The reason that
this did not cause a problem here is because in 122923 the check in __ast_read
to check the exception flag was removed.

(related to #18637)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.6.2@306123 65c4cc65-6c06-0410-ace0-fbb531ad65f3