[Events] Lower priority of Event Script process after fork()

Event Scripts were running with the same elevated priority as the
rest of FPP which could cause FPP and audio playback to hickup
when an Event Script was run.  Especially noticable when running
a perl script in an Event Script.
(cherry picked from commit 40982d9165698cfd99dccc777372c8f6c5b6ede4)