Add a routine to report write errors to the list of failure-reporting

Add a routine to report write errors to the list of failure-reporting
routines handled by epan/report_err.c.

Move copy_binary_file() in file.c to epan/filesystem.c, and rename it to
copy_file_binary_mode() (to clarify that it *can* copy text files;
arguably, *all* files are "binary" unless you're on, say, an IBM 1401
:-)).  Have it use the report_err.c routines, so it works in
console-mode programs.

Clean up some comments while we're at it.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@27456 f5534014-38df-0310-8fa8-9805f1628bb7