Fix: Use 'subnet_of' instead of 'overlaps' for reserved IP range check

[ipspace]

The checks of reserved IP ranges used 'overlaps' function which triggered an error even when '0.0.0.0/0' was compared to '0.0.0.0/8'

Fixes #3214

[Copilot]

Pull request overview

Fixes IPv4 reserved-range validation so that supernets like 0.0.0.0/0 are not incorrectly rejected as “in” a smaller reserved block, aligning bgp.originate behavior with expected default-route use cases (issue #3214).

Changes:

• Switch reserved-range detection from overlaps() to subnet_of() to avoid false positives for supernets (e.g. 0.0.0.0/0).
• Extend IPv4 reserved ranges to include Class E (240.0.0.0/4).

[jbemmel]

I think a regression test case for bgp.originate: 0.0.0.0/0 would be good; we may also want to consider deprecating bgp.session default_originate, in favor of this more intuitive (imho) version

[ipspace]

we may also want to consider deprecating bgp.session default_originate, in favor of this more intuitive (imho) version

We need both (there's a reason many devices support both ;) -- you might want to advertise a default route without having it, and you might want to advertise it only to some neighbors (customers but not peers)