net: fix parsing literal IP addresses in local database

net: fix parsing literal IP addresses in local database

This change fixes incorrect parsing of literal IP addresses in local
database when the addresses contain IPv6 zone identifiers, are in
dotted-decimal notation or in colon-hexadecimal notation with leading
zeros.

https://golang.org/cl/5851 already fixed the code path using getaddrinfo
via cgo. This change fixes the remaining non-cgo code path.

Fixes #8243.
Fixes #8996.

Change-Id: I48443611cbabed0d69667cc73911ba3de396fd44
Reviewed-on: https://go-review.googlesource.com/10306
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>