check_apt bugfix: now checking if the apt-get binary is readable

Hi Nagios Plugins Team,
here's a pull request which fixed an strange error message and an OK return code if no apt-get binary is available when using check_apt.
The message looked like:
```
$ ./check_apt 
' -o 'Debug::NoLocking=true' -s -qq upgrade' exited with non-zero status.
APT OK: 0 packages available for upgrade (0 critical updates).  errors detected. run with -v for information.
$ echo $?
0
```

After this patch check_apt will cleanly exit with an UNKNOWN state:
```
$ ./check_apt 
APT UNKNOWN: unable to open apt-get binary (/usr/bin/apt-get)
$ echo $?
3
```

If there are any questions please don't hesitate to contact me.

regards,
Richard