libc/resolv: Reimplement the sortlist parser

[des]

Summary

When we switched from the BIND4 resolver to the BIND9 resolver, the
sortlist parser was inadvertently disabled, and nobody noticed. The
sorting code remained enabled in the resolver, but there was no way to
set a sort order.

Reimplement the sortlist parser, but correctly, and update the
documentation accordingly. The new parser accepts IPv4 and IPv6
addresses with or without a prefix length.

Fixes: 5342d17f09a8 ("Update the resolver in libc to BIND9's one.")
Relnotes: yes