netstat: Resolve duplicate "dropped-packets" key from libxo output

The current libxo output uses the "dropped-packets" key for both incoming and
outgoing packets. This commit distinguishes between the two by adding an "in"
and "out" suffix. The original "dropped-packets" key is kept for backwards
compatibility for now.

PR: 244589
Reviewed by: imp, zlei
Pull Request: https://github.com/freebsd/freebsd-src/pull/1331