Support unixgram syslog address

- Consider unixgram:// as a valid URL prefix
- Parse unixgram:// addresses
- Update docs

Signed-off-by: Zhuoyun Wei <wzyboy@wzyboy.org>

urlutil/urlutil.go

@@ -11,7 +11,7 @@ var (
 	validPrefixes = map[string][]string{
 		"url":       {"http://", "https://"},
 		"git":       {"git://", "github.com/", "git@"},
-		"transport": {"tcp://", "tcp+tls://", "udp://", "unix://"},
+		"transport": {"tcp://", "tcp+tls://", "udp://", "unix://", "unixgram://"},
 	}
 	urlPathWithFragmentSuffix = regexp.MustCompile(".git(?:#.+)?$")
 )

urlutil/urlutil_test.go

@@ -23,6 +23,7 @@ var (
 		"tcp+tls://example.com",
 		"udp://example.com",
 		"unix:///example",
+		"unixgram:///example",
 	}
 )