git: rc.d/sysctl: Rewrite to be more robust and clean

[Aaron LI]

Quoting Sepherosa Ziehau (2019-02-01 21:33:18)
> Please check fix this commit, there are several unexpected effects of
> this commit:
> - Extra original value printed on the screen, when this script is run.
> Before this change, a=`cmd` will not print anything on the screen.
> - It looks after or during "TCP options" sysctl, this script is
> invoked again in a quite verbose mode, which does not happen before.
> 

Hi sephe, thanks for pointing out the issues.

Could you post your /etc/sysctl.conf so that I can see the exact 
unexpected behaviors?  Thanks.

Aaron

PS. I'll look into the issues when I get home on Feb 3.