Audit errno (ab)uses

[evelikov]

As mentioned in #240 there are places were we set errno only to never check it. Glancing around we also have instances that set it, where it can be returned directly via the function.

So let try to:

• list all instances which set errno
• remove instances which are not used by libkmod nor libc
• use a simpler way to propagate the error, if available