redsocks.c: In function 'redsocks_read_expected':
redsocks.c:407: warning: unused variable 'read'
http-relay.c: In function 'httpr_client_read_cb':
http-relay.c:520: warning: unused variable 'written_wo_null'
These variables are used in asserts and if asserts are not compiled,
e.g. by defining NDEBUG (usually in CFLAGS via -DNDEBUG), then we have
misleading warnings.
Add in utils.h UNUSED() macro for creating null statement with result
casted to void. Use it to fix above warnings.
