tree-wide: make parse_ifindex simply return the index

tree-wide: make parse_ifindex simply return the index

We don't need a seperate output parameter that is of type int.  glibc() says
that the type is "unsigned", but the kernel thinks it's "int".  And the
"alternative names" interface also uses ints. So let's standarize on ints,
since it's clearly not realisitic to have interface numbers in the upper half
of unsigned int range.