lib: handle failure to change ifindex

lib: handle failure to change ifindex

This fixes a theoretical bug that could occur when trying to change an
ifindex on an interface to that of an existing interface. We would
remove the interface from the ifindex tree, and change the ifindex, but
when we tried to reinsert the interface, the insert would fail. It was
impossible to know if this failed due to the insertion / deletion macros
capturing the result value of the underlying BSD tree macros. So we
would effectively delete the interface.

Instead of failing on insert, we just check if the prospective ifindex
already exists and return -1 if it does.

Macros have been changed to statement expressions so the result can be
checked, and bubbled up.

Signed-off-by: Quentin Young <qlyoung@cumulusnetworks.com>