Split rtrequest1_fib() into smaller manageable chunks.

No functional changes.

• Move route addition / route deletion code from rtrequest1_fib() to add_route() and del_route() respectively.
• Rename rtrequest1_fib_change() to change_route() for consistency.
• Shrink the scope of ugly info #defines.

MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D24349