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 <e7e312fea4c2c1aad2bb075d739111890e1ce08b@zx2c4.com>
Trust: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Jason A. Donenfeld <e7e312fea4c2c1aad2bb075d739111890e1ce08b@zx2c4.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

nawawi/go,cloudflare/go,nawawi/go,cloudflare/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,nawawi/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,golang/go,cloudflare/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,golang/go,CAFxX/go,golang/go,golang/go,CAFxX/go,cloudflare/go,nawawi/go