shellcheck: nagios-check-tor-authority-cert issues

In nagios-check-tor-authority-cert line 52:
TMPFILE="`tempfile`"
         ^-- SC2006: Use $(..) instead of legacy `..`.
          ^-- SC2186: tempfile is deprecated. Use mktemp instead.


In nagios-check-tor-authority-cert line 57:
	if [ "$?" = 0 ]; then
             ^-- SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.


In nagios-check-tor-authority-cert line 77:
elif [ "$(( $now + 7*24*60*60 ))" -ge "$expiryunix" ]; then
            ^-- SC2004: $/${} is unnecessary on arithmetic variables.


In nagios-check-tor-authority-cert line 80:
elif [ "$(( $now + 30*24*60*60 ))" -ge "$expiryunix" ]; then
            ^-- SC2004: $/${} is unnecessary on arithmetic variables.