ldpd: During code inspection we are mixing data sizes

ldpd: During code inspection we are mixing data sizes

As I understand it ldpd was originally developed as a standalone
daemon for *BSD land.  Then ported to FRR.  FRR uses ifindex_t
as the base type for the ifindex.  Mixing `unsigned short` and
`int` and `unsigned int` is going to lead to fun somewhere
along the way.  Especially when we get to run on a system
with ifindex churn( I'm looking at you docker ).

Attempt to convert all of ldpd to think of the ifindex as a
`ifindex_t`.

Signed-off-by: Donald Sharp <sharpd@cumulusnetworks.com>