Increase the file descriptor limit before extracting

We got a [report](https://discourse.appimage.org/t/running-appimage-app-on-ubuntu-16-04/200/6?u=probono) that `--appimage-extract` fails on a very large (15 GB) file and that maybe we are running out of file descriptors.

https://github.com/ipfs/go-ipfs seems to be able to increase the file descriptor limit without needing root rights, maybe we could do the same:

```
me@host:~$ '/home/me/ipfs' daemon
Initializing daemon...
Adjusting current ulimit to 2048...
Successfully raised file descriptor limit to 2048.
```

The code doing this is in https://github.com/ipfs/go-ipfs/blob/master/cmd/ipfs/ulimit_unix.go