Add rolling log file.

This commit modifies the logging to also log all output to a rolling log
file in the btcd home directory under the logs folder.  It uses a maximum
size of 10MB per log file and a max rotation size of 3.  This means the
log files will not exceed 30 megabytes.

config.go

@@ -39,6 +39,7 @@ var (
 	knownDbTypes       = btcdb.SupportedDBs()
 	defaultRPCKeyFile  = filepath.Join(btcdHomeDir, "rpc.key")
 	defaultRPCCertFile = filepath.Join(btcdHomeDir, "rpc.cert")
+	defaultLogFile     = filepath.Join(btcdHomeDir, "logs", "btcd.log")
 )
 
 // config defines the configuration options for btcd.

log.go

@@ -130,12 +130,14 @@ func newSeelogLogger() seelog.LoggerInterface {
 	<seelog type="adaptive" mininterval="2000000" maxinterval="100000000"
 		critmsgcount="500" minlevel="trace">
 		<outputs formatid="all">
-			<console/>
+			<console />
+			<rollingfile type="size" filename="%s" maxsize="10485760" maxrolls="3" />
 		</outputs>
 		<formats>
-			<format id="all" format="%Time %Date [%LEV] %Msg%n" />
+			<format id="all" format="%%Time %%Date [%%LEV] %%Msg%%n" />
 		</formats>
 	</seelog>`
+	config = fmt.Sprintf(config, defaultLogFile)
 
 	logger, err := seelog.LoggerFromConfigAsString(config)
 	if err != nil {