MacOS/BSD compatiblity

[neocturne]

Let's hope there aren't any more incompatible qsort_r definitions...

[tcatm]

exit_error() should still allow format strings like error() does.

[neocturne]

I tried not to introduce unnecessary code, the usage message was the only place where the format string was actually used.

Would you be okay with a macro definition, like in http://git.openwrt.org/?p=openwrt.git;a=commitdiff;h=76dde10b2061b8fedf88c38712fc054219a934d9 ?

[tcatm]

The macro looks fine.

[neocturne]

Updated. I've found an issue with the macro (it doesn't to the right thing when errnum == 0), so I've replaced it with a proper function.