crypto/x509: allow ":" in Common Name hostnames

At least one popular service puts a hostname which contains a ":"
in the Common Name field. On the other hand, I don't know of any name
constrained certificates that only work if we ignore such CNs.

Updates #24151

Change-Id: I2d813e3e522ebd65ab5ea5cd83390467a869eea3
Reviewed-on: https://go-review.googlesource.com/134076
Run-TryBot: Filippo Valsorda <filippo@golang.org>
Reviewed-by: Adam Langley <agl@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/crypto/x509/verify.go

@@ -894,8 +894,8 @@ func validHostname(host string) bool {
 			if c == '-' && j != 0 {
 				continue
 			}
-			if c == '_' {
-				// _ is not a valid character in hostnames, but it's commonly
+			if c == '_' || c == ':' {
+				// Not valid characters in hostnames, but commonly
 				// found in deployments outside the WebPKI.
 				continue
 			}

src/crypto/x509/verify_test.go

@@ -1881,6 +1881,7 @@ func TestValidHostname(t *testing.T) {
 		{"foo.*.example.com", false},
 		{"exa_mple.com", true},
 		{"foo,bar", false},
+		{"project-dev:us-central1:main", true},
 	}
 	for _, tt := range tests {
 		if got := validHostname(tt.host); got != tt.want {