rasdaemon: add mc_event trigger

rasdaemon: add mc_event trigger

Allow users to run a trigger when RAS mc_event occurs, The mc_event
trigger is separated into CE trigger and UE trigger, this is because
CE is more frequent than UE, and the CE trigger will lead to more
performance hits. Users can choose different triggers for CE/UE to
reduce this effect.

Users can config trigger in /etc/sysconfig/rasdaemon:

    TRIGGER_DIR: The trigger diretory
    MC_CE_TRIGGER: The script executed when corrected error occurs.
    MC_UE_TRIGGER: The script executed when uncorrected error occurs.

No script will be executed if MC_CE_TRIGGER/MC_UE_TRIGGER is null.

Signed-off-by: Ruidong Tian <tianruidong@linux.alibaba.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>