ndp: fix macOS IPv6 compatibility by using link-local source addresses

ndp: fix macOS IPv6 compatibility by using link-local source addresses

macOS ignores NDP packets that don't originate from link-local addresses,
causing IPv6 connectivity issues with odhcpd. This change ensures NDP
packets (Neighbor Advertisements and ICMP Echo Requests) are sent using
link-local source addresses for RFC 4861 compliance.

Changes:
* Add ndp_from_link_local configuration flag (defaults to true)
* Add odhcpd_send_with_src() to allow explicit source address control
* Add odhcpd_try_send_with_src() helper to eliminate code duplication
* Add odhcpd_get_interface_linklocal_addr() with caching for performance
* Update send_na() and ping6() to use link-local source addresses when
  enabled
* Add RFC 4861, §4.2 comments explaining the mandated behavior
* Maintain backward compatibility with fallback behavior

Fixes: openwrt/openwrt#7561 #202
Signed-off-by: Stephen Groat <[email protected]>
Link: https://github.com/openwrt/odhcpd/pull/242
Signed-off-by: Álvaro Fernández Rojas <[email protected]>