Move the logeventf wrappers into their own source file.

Separating them from logging.c allows them to be shared between the
real logging.c and the new stub no-logging.c.

utils/logeventf.c

@@ -0,0 +1,32 @@
+/*
+ * Helpful wrapper functions around the raw logevent().
+ *
+ * This source file lives in 'utils' because it's conceptually a
+ * convenience utility rather than core functionality. But it can't
+ * live in the utils _library_, because then it might refer to
+ * logevent() in an earlier library after Unix ld had already finished
+ * searching that library, and cause a link failure. So it must live
+ * alongside logging.c.
+ */
+
+#include "putty.h"
+
+void logevent_and_free(LogContext *ctx, char *event)
+{
+    logevent(ctx, event);
+    sfree(event);
+}
+
+void logeventvf(LogContext *ctx, const char *fmt, va_list ap)
+{
+    logevent_and_free(ctx, dupvprintf(fmt, ap));
+}
+
+void logeventf(LogContext *ctx, const char *fmt, ...)
+{
+    va_list ap;
+
+    va_start(ap, fmt);
+    logeventvf(ctx, fmt, ap);
+    va_end(ap);
+}