pipe_progress: make it independent of printf machinery

function                                             old     new   delta
bb_putchar_stderr                                      -      24     +24
ParseField                                           494     471     -23
progress_meter                                       212     188     -24
xargs_main                                           888     842     -46
pipe_progress_main                                   151     105     -46
------------------------------------------------------------------------------
(add/remove: 2/0 grow/shrink: 0/4 up/down: 24/-139)          Total: -115 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/include/libbb.h b/include/libbb.h
index 326179b..3fffa83 100644
--- a/include/libbb.h
+++ b/include/libbb.h
@@ -592,6 +592,8 @@
 /* Guaranteed to NOT be a macro (smallest code). Saves nearly 2k on uclibc.
  * But potentially slow, don't use in one-billion-times loops */
 int bb_putchar(int ch) FAST_FUNC;
+/* Note: does not use stdio, writes to fd 2 directly */
+int bb_putchar_stderr(char ch) FAST_FUNC;
 char *xasprintf(const char *format, ...) __attribute__ ((format(printf, 1, 2))) FAST_FUNC RETURNS_MALLOC;
 // gcc-4.1.1 still isn't good enough at optimizing it
 // (+200 bytes compared to macro)