Change some of the merge.c APIs to return more information on failure,

Change some of the merge.c APIs to return more information on failure,
and use that information to provide better error messages.

Have "merge_open_outfile()" do all the work of filling in the
merge_out_file_t structure, with the values to use passed as arguments.
Get rid of some structure members that used to be used solely to pass
information to "merge_open_outfile()".

Add a "cf_merge_files()" routine to do the merging and reporting of errors.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@12420 f5534014-38df-0310-8fa8-9805f1628bb7