net/route.h: Expose the ROUNDUP and ADVANCE macros.

* These macros are replicated in multiple places on the tree.  Give
  them an RT_ prefix and centralize them in net/route.h in an effort
  to reduce code duplication.

* Kernel and userland changes to use these macros will come in a
  subsequent commit.

Taken-from: NetBSD
Reviewed-by: dillon
Use system's RT_ROUNDUP and RT_ADVANCE macros instead of local copies.

Reviewed-by: dillon