net/netip: add IPv4Unspecified

net/netip: add IPv4Unspecified

There is IPv6Unspecified but there is not IPv4Unspecified, making for
inconsistent code. This commit adds the missing function.

Updates #49298.

Change-Id: Id2519b646323642f59fb1cc6ea8e335fdde16290
Reviewed-on: https://go-review.googlesource.com/c/go/+/361056
Trust: Jason A. Donenfeld <Jason@zx2c4.com>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Jason A. Donenfeld <Jason@zx2c4.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>