insmod: Fix exit code regression

[stoeckmann]

The tools have two different ways of deciding how the final exit code should look like:

1. Variable r is set and checked
2. Variable err is set and checked

The commit ca8f04e adjusted insmod, expecting that option 1 is in place. But insmod belongs to the group which uses option 2.

Since this commit never hit any release, this regression never affected anyone out there (I guess).

Proof of Concept:

kmod 33:

$ insmod -
insmod: ERROR: this tool does not support loading from stdin!
$ echo $?
1

kmod master:

$ insmod -
insmod: ERROR: this tool does not support loading from stdin!
$ echo $?
0

[evelikov]

There is an alternative fix following Lucas' suggestion in #172

Let me go around and pull the prep work to separate PR(s)...