loop on fflush()->EAGAIN

sometimes when we call fflush(ttyfp), we get EAGAIN. we can
loop on this and force the flush. without it, we (fairly rarely)
get errors on shutdown, sometimes manifesting as unit test
failures (especially in the rust wrappers). #742

src/lib/internal.h

@@ -506,16 +506,20 @@ term_emit(const char* name __attribute__ ((unused)), const char* seq,
   }
   int ret = fprintf(out, "%s", seq);
   if(ret < 0){
-// fprintf(stderr, "Error emitting %zub %s escape (%s)\n", strlen(seq), name, strerror(errno));
+//fprintf(stderr, "Error emitting %zub %s escape (%s)\n", strlen(seq), name, strerror(errno));
     return -1;
   }
   if((size_t)ret != strlen(seq)){
-// fprintf(stderr, "Short write (%db) for %zub %s sequence\n", ret, strlen(seq), name);
+//fprintf(stderr, "Short write (%db) for %zub %s sequence\n", ret, strlen(seq), name);
     return -1;
   }
-  if(flush && fflush(out)){
-// fprintf(stderr, "Error flushing after %db %s sequence (%s)\n", ret, name, strerror(errno));
-    return -1;
+  if(flush){
+    while(fflush(out) == EOF){
+      if(errno != EAGAIN){
+        fprintf(stderr, "Error flushing after %db %s sequence (%s)\n", ret, name, strerror(errno));
+        return -1;
+      }
+    }
   }
   return 0;
 }