* Try to obtain FTL's PID from the PID file. If this fails, try to identify the main process using pgrep --oldest (instead of relying on pkill finding the right one by itself). This allows the script to work in even when FTL is running inside the memory checker valgrind.
* Rename FTL_PID -> FTL_PID_FILE
* Remove the pgrep fallback after discussions about that it should be more obvious to users if something strange happened to their PID file. Also, simplify the routine using a bashism in the end.
* Shorten if [[ regex ]]
* Use unset instead of emptying the PID variable
Signed-off-by: DL6ER <dl6er@dl6er.de>
Co-authored-by: Dan Schaper <dan.schaper@pi-hole.net>