Treat core_count as integer

[yubiuser]

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• What does this PR aim to accomplish?:

Fixes #224.

PR #212 removed the declare -i core_count because declare is not supported by POSIX. However, later we do an arithmetic +1 to core_count which will not be interpreted as addition but results in something like 255+1. This is not a visible issue, unless cat /sys/devices/system/cpu/kernel_max returns 0 which leads to the issue seen in #224 (division by zero resulting in inf by awk)

• How does this PR accomplish the above?:

Use $(( )) syntax to do arithmetic expansion.

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[dschaper]

Was there a reason for that sysfs file being the one? Or it's what was there before.

$ cat /sys/devices/system/cpu/possible
0-31

$ cat /sys/devices/system/cpu/present
0-15

$ cat /sys/devices/system/cpu/online
0-15

$ cat /sys/devices/system/cpu/offline 
16-31

From https://unix.stackexchange.com/questions/609555/how-does-linux-detect-number-of-possible-cpus

[rdwebdesign]

Was there a reason for that sysfs file being the one? Or it's what was there before.

It was this file for the last 4 years:
https://github.com/pi-hole/PADD/blame/ca45bbda551107b6a4c4a88aed5d9a7513942cef/padd.sh#L25

Introduced here: #37

[dschaper]

What does /sys/devices/system/cpu/kernel_max show on the RPi? I think it's almost always going to be 255.

[yubiuser]

Alternative proposal: #227

[yubiuser]

Closing this as #227 has been merged.