Use vsnprintf(3) in syslog(3).

It seemed like a clever trick to use the internal log message formatting
code in syslog(3), but on reflection that means you can't (for example)
format floating point numbers. This patch switches us over to using good
old vsnprintf(3), even though that requires us to jump through a few hoops.

There's no obvious way to unit test this, so I wrote a little program and
ran that.

Bug: 14292866
Change-Id: I9c83500ba9cbb209b6f496067a91bf69434eeef5

libc/bionic/libc_logging.cpp

@@ -230,9 +230,6 @@ static void SendRepeat(Out& o, char ch, int count) {
 /* Perform formatted output to an output target 'o' */
 template <typename Out>
 static void out_vformat(Out& o, const char* format, va_list args) {
-#if 0
-    int caller_errno = errno;
-#endif
     int nn = 0;
 
     for (;;) {
@@ -381,11 +378,6 @@ static void out_vformat(Out& o, const char* format, va_list args) {
         } else if (c == '%') {
             buffer[0] = '%';
             buffer[1] = '\0';
-#if 0
-        } else if (c == 'm') {
-            // syslog-like %m for strerror(errno).
-            str = strerror(caller_errno);
-#endif
         } else {
             __assert(__FILE__, __LINE__, "conversion specifier unsupported");
         }