log: Replace glog with a new logging module

glog works fine and has great features, but it does not play along well
with systemd or standard log rotators (as it does the rotation itself).

So this patch replaces glog with a new logging module "log", which by
default logs to stderr, in a systemd-friendly manner.

Logging to files or syslog is still supported.

internal/smtpsrv/server_test.go

@@ -20,8 +20,6 @@ import (
 	"blitiri.com.ar/go/chasquid/internal/aliases"
 	"blitiri.com.ar/go/chasquid/internal/courier"
 	"blitiri.com.ar/go/chasquid/internal/userdb"
-
-	"github.com/golang/glog"
 )
 
 // Flags.
@@ -414,7 +412,6 @@ func waitForServer(addr string) error {
 // os.Exit(). We have to do this so we can use defer.
 func realMain(m *testing.M) int {
 	flag.Parse()
-	defer glog.Flush()
 
 	if *externalSMTPAddr != "" {
 		smtpAddr = *externalSMTPAddr