Further refactor IPv4 interface route creation.

• Fix bug with /32 aliases introduced in 81728a538d24.
• Explicitly document business logic for IPv4 ifa routes.
• Remove remnants of rtinit()
• Deduplicate ifa->route prefix code by moving it into ia_getrtprefix()
• Deduplicate conditional check for ifa_maintain_loopback_route() by moving into ia_need_loopback_route()
• Remove now-unused flags argument from in_addprefix().

Reviewed by: donner
PR: 252883
Differential Revision: https://reviews.freebsd.org/D28246