init: make LIBISCSI_CACHE_ALLOCATIONS a public environment variable

this variable was introduced for iscsi-test-cu only. This patch
makes it a generic environment variable that can be set per context.

Signed-off-by: Peter Lieven <pl@kamp.de>

include/iscsi.h

@@ -51,7 +51,7 @@ struct sockaddr;
   "<host>[:<port>]\""
 
 
-EXTERN void iscsi_set_cache_allocations(int ca);
+EXTERN void iscsi_set_cache_allocations(struct iscsi_context *iscsi, int ca);
 
 /*
  * The following three functions are used to integrate libiscsi in an event