We should be *sourcing* `/etc/sysconfig/gogs`, not *executing* it, don't we?

This change to start.sh allows it to be executed from any working directory and always reference the gogs binary in the parent directory. Very helpful when calling start.sh from an init script.

fix

how to use:
vim /etc/rc.local, then add
cd /home/git/gogs; scripts/autoboot.sh
(goes path you can replace your path)

now, we auto add the start.sh -> /usr/bin/gogs_start, then supervisor
needn’t the full path, detail please see the commit.
and how to use:
entry gogs root path, then input
scripts/gogs_supervisord.sh restart (add sudo if need)

Implies 2 running child, so the `stop` command usually needs a `KILL` instead of a `TERM` to close properly so I reduced it to 1 second to get quickly with the `KILL` signal.

Compatible with "/etc/default/gogs" configuration file for adding or overwriting values.
Based on "/etc/init.d/skeleton" and tested on Raspbian 2014-06-20 (Debian 7.5 [Wheezy] ARM)

[TO FIX] For some reason, it's necessary to remove the value of RUN_USER from the app.ini configuration file because the following error occurs:
2014/09/28 18:12:53 [setting.go:182 NewConfigContext()] [E] Expect user(git) but current user is:

Removed trailing whitespaces with

```sh
for file in `grep -r " $" * | cut -d : -f 1 | sort | uniq | grep -v " "`; do sed -i 's/[ \t]*$//' $file; done
```

and some handwork.