mobile_ppp: Don't pass invalid arguments to kill command

#### Description
Stopping mobile PPP connections does not work right now because invalid arguments are passed to the `kill` command.

In the file [src/lib/connections/mobile_ppp](https://github.com/joukewitteveen/netctl/blob/master/src/lib/connections/mobile_ppp) the function `mobile_ppp_down()` is called to kill the PPP connection.

The following command is used to kill pppd (line 87 in current master):
```bash
[[ -r $pidfile ]] && kill "$(< "$pidfile")"
```
The pid file looks like this:
```bash
cat /var/run/ppp-mobile.pid
6104
ppp0
```

The log tells me:
```bash
journalctl -xn
Mar 12 17:36:57 host network[3141]: /usr/lib/network/connections/mobile_ppp: line 87: kill: 2879
Mar 12 17:36:57 host network[3141]: ppp0: arguments must be process or job IDs
```
The kill command does not work becuase it contains more than just the PID. Currently the complete file is passed.

#### Fix
This pull request fixes this issue, it only passes the first line of the PID file to the kill command.

#### Version info
```bash
pppd --version
pppd version 2.4.6
```