Shellvars: Allow (almost) any command

This PR intends to parse more shellvar files by trying to parse most commands.

E.g.:

```shell
$ cat /usr/local/bin/sh2js 
#!/bin/sh
cd /opt/shjs-0.6/
./sh2js.pl $@

$ augtool -I lenses/ --noautoload -t "Shellvars.lns incl /usr/local/bin/sh2js" print /files
/files
/files/usr
/files/usr/local
/files/usr/local/bin
/files/usr/local/bin/sh2js
/files/usr/local/bin/sh2js/#comment = "!/bin/sh"
/files/usr/local/bin/sh2js/@command[1] = "cd"
/files/usr/local/bin/sh2js/@command[1]/@arg = "/opt/shjs-0.6/"
/files/usr/local/bin/sh2js/@command[2] = "./sh2js.pl"
/files/usr/local/bin/sh2js/@command[2]/@arg = "$@"
```