A few new minor utility functions.

A function to compare two strings _both_ in ptrlen form (I've had
ptrlen_eq_string for ages, but for some reason, never quite needed
ptrlen_eq_ptrlen). A function to ask whether one ptrlen starts with
another (and, optionally, return a ptrlen giving the remaining part of
the longer string). And the va_list version of logeventf, which I
really ought to have written in the first place by sheer habit, even
if it was only needed by logeventf itself.

logging.c

@@ -256,15 +256,18 @@ void logevent_and_free(LogContext *ctx, char *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;
-    char *buf;
 
     va_start(ap, fmt);
-    buf = dupvprintf(fmt, ap);
+    logeventvf(ctx, fmt, ap);
     va_end(ap);
-    logevent_and_free(ctx, buf);
 }
 
 /*