basic: add explicit ipv4-specific in_addr classification calls

basic: add explicit ipv4-specific in_addr classification calls

This adds in4_addr_is_localhost() and in4_addr_is_link_local() that only take
an IPv4 "struct in_addr", to match in_addr_is_localhost() and
in_addr_is_link_local() that that a "union in_addr_union".

This matches the existing in4_addr_is_null() call that already exists.

For IPv6 glibc already exports a set of macros, hence we don't add similar
functions in6_addr_is_localhost(). We also drop in6_addr_is_null() as
IN6_IS_ADDR_UNSPECIFIED() already provides that.