libflux: hide internal log buffer size from public API

Problem: libflux/flog.h exposes FLUX_MAX_LOGBUF via the public API,
but there are currently no users. Furthermore, the implementation
now falls back to a heap-allocated buffer if the internal fixed-size
buffer causes log message truncation, so exposing the internal buffer
size is misleading.

Move the definition of FLUX_MAX_LOGBUF out of the public API.

src/common/libflux/flog.c

@@ -25,6 +25,8 @@
 #include "src/common/libutil/wallclock.h"
 #include "src/common/libutil/stdlog.h"
 
+#define FLUX_MAX_LOGBUF     2048
+
 typedef struct {
     char appname[STDLOG_MAX_APPNAME + 1];
     char procid[STDLOG_MAX_PROCID + 1];

src/common/libflux/flog.h

@@ -20,9 +20,6 @@
 extern "C" {
 #endif
 
-
-#define FLUX_MAX_LOGBUF     2048
-
 typedef void (*flux_log_f)(const char *buf, int len, void *arg);
 
 /* Set log appname for handle instance.