MINOR: compression: CPU usage limit

New option 'maxcompcpuusage' in global section.
Sets the maximum CPU usage HAProxy can reach before stopping the
compression for new requests or decreasing the compression level of
current requests.  It works like 'maxcomprate' but with the Idle.
diff --git a/include/proto/compression.h b/include/proto/compression.h
index 0eaa216..a2e80b8 100644
--- a/include/proto/compression.h
+++ b/include/proto/compression.h
@@ -25,6 +25,8 @@
 
 #include <types/compression.h>
 
+extern unsigned int compress_min_idle;
+
 int comp_append_type(struct comp *comp, const char *type);
 int comp_append_algo(struct comp *comp, const char *algo);
 
@@ -40,6 +42,7 @@
 int identity_end(struct comp_ctx **comp_ctx);
 
 
+
 #ifdef USE_ZLIB
 extern long zlib_used_memory;