feat: implement syslog logging (#181)

* feat: implement syslog logging

With this functionality in tree, macOS users could easily access
ooniprobe logs by filtering by process name in the console app.

Part of https://github.com/ooni/probe/issues/1289

* fix: build on windows

* fix: all build issues

internal/log/handlers/syslog/syslog.c

@@ -0,0 +1,39 @@
+#ifndef _WIN32
+#include <syslog.h>
+#endif
+
+void ooniprobe_openlog(void) {
+#ifndef _WIN32
+    (void)openlog("ooniprobe", LOG_PID, LOG_USER);
+#endif
+}
+
+void ooniprobe_log_debug(const char *message) {
+#ifndef _WIN32
+    (void)syslog(LOG_DEBUG, "%s", message);
+#endif
+}
+
+void ooniprobe_log_info(const char *message) {
+#ifndef _WIN32
+    (void)syslog(LOG_INFO, "%s", message);
+#endif
+}
+
+void ooniprobe_log_warning(const char *message) {
+#ifndef _WIN32
+    (void)syslog(LOG_WARNING, "%s", message);
+#endif
+}
+
+void ooniprobe_log_err(const char *message) {
+#ifndef _WIN32
+    (void)syslog(LOG_ERR, "%s", message);
+#endif
+}
+
+void ooniprobe_log_crit(const char *message) {
+#ifndef _WIN32
+    (void)syslog(LOG_CRIT, "%s", message);
+#endif
+}

internal/log/handlers/syslog/syslog.go

@@ -0,0 +1,53 @@
+// Package syslog contains a syslog handler.
+//
+// We use this handler on macOS systems to log messages
+// when ooniprobe is running in the background.
+package syslog
+
+import (
+	"fmt"
+	"unsafe"
+
+	"github.com/apex/log"
+)
+
+/*
+#include<stdlib.h>
+
+void ooniprobe_openlog(void);
+void ooniprobe_log_debug(const char *message);
+void ooniprobe_log_info(const char *message);
+void ooniprobe_log_warning(const char *message);
+void ooniprobe_log_err(const char *message);
+void ooniprobe_log_crit(const char *message);
+*/
+import "C"
+
+// Default is the handler that emits logs with syslog
+var Default log.Handler = newhandler()
+
+type handler struct{}
+
+func newhandler() handler {
+	C.ooniprobe_openlog()
+	return handler{}
+}
+
+func (h handler) HandleLog(e *log.Entry) error {
+	message := fmt.Sprintf("%s %+v", e.Message, e.Fields)
+	cstr := C.CString(message)
+	defer C.free(unsafe.Pointer(cstr))
+	switch e.Level {
+	case log.DebugLevel:
+		C.ooniprobe_log_debug(cstr)
+	case log.InfoLevel:
+		C.ooniprobe_log_info(cstr)
+	case log.WarnLevel:
+		C.ooniprobe_log_warning(cstr)
+	case log.ErrorLevel:
+		C.ooniprobe_log_err(cstr)
+	default:
+		C.ooniprobe_log_crit(cstr)
+	}
+	return nil
+}