[nghttp] copy the request entity into temporary file if necessary

This pull-request changes the `nghttp` command to copy the request entity designated by `-d` option, if the value is `-` (i.e. stdin) and if it is not a regular file.  It is necessary since stdin might not be seekable, while the input needs to be read more than once if `-m` option is used.

Note: the implemetation is based on the design discussed in https://github.com/tatsuhiro-t/nghttp2/issues/133#issuecomment-67747021
Note 2: I have read the contribution guidelines and I believe that this PR does not violate the described rules.