mch_write -> term_write

Switch from POSIX's write() to fwrite(stdout,...) and disable buffering
since vim buffers output explicitly and flushes when needed, like when a
key is typed.

src/nvim/ui.c

@@ -55,7 +55,7 @@ void ui_write(char_u *s, int len)
         s = tofree;
     }
 
-    mch_write(s, len);
+    term_write(s, len);
 
     if (output_conv.vc_type != CONV_NONE)
       free(tofree);