Have "cf_merge_files()" take a pointer-to-pointer-to-char as the output

Have "cf_merge_files()" take a pointer-to-pointer-to-char as the output
file name argument; if the pointed-to pointer is null, it opens a
temporary file, and sets that pointer to a mallocated copy of the
pathname of the temporary file.  It no longer needs a file descriptor as
an argument.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@13419 f5534014-38df-0310-8fa8-9805f1628bb7