nginx: copies the req body chain to be processed instead of move

Add a check for the definition MOVE_REQUEST_CHAIN_TO_MODSEC, whenever it is
set the chain will be moved into the brigade. If it was not set the chain
will be only copied. Moving was causing segfaults on the following
regression tests:

 #15 - SecRequestBodyInMemoryLimit
 #16 - SecRequestBodyInMemoryLimit (greater)
 #19 - SecRequestBodyLimitAction ProcessPartial (multipart/greater - chunked)
 (from: regression/config/10-request-directives.t)