lavu: add av_usleep() function

This function implements a delay using the first available
of the following functions:

- nanosleep()
- usleep()
- Sleep() (Windows)

The conditional #includes in time.c are simplified by including
unistd.h and windows.h whenever they are available rather than
having these lines triggered by specific functions.

Signed-off-by: Mans Rullgard <mans@mansr.com>

doc/APIchanges

@@ -13,6 +13,9 @@ libavutil:     2011-04-18
 
 API changes, most recent first:
 
+2012-06-22 - xxxxxxx - lavu 51.34.0
+  Add av_usleep()
+
 2012-06-20 - ae0a301 - lavu 51.33.0
   Move av_gettime() to libavutil, add libavutil/time.h