usr_bin/random-hostname: Fix a couple of shellcheck warnings

usr_bin/random-hostname: Fix a couple of shellcheck warnings

- Ignore SC2034: hostnames appears unused. Verify use (or export if used externally).

While at it I rewrote the way how the random hostname is retrieved:

First find the line number 3 lines after the line which starts with "# EOF"
This is the line where the list of hostnames start.
This makes the script a bit more robust and avoids the hard coded start
line calculation ("num=$(( $num - 16 ))").

Then list all lines from there by excluding all empty commented out
lines and the last line (which start with "'").
Use "sort -R" to randomly sort the list and from there take the last line.

That way the confusing "randline" variable calculation is avoided.