Pass ESD_BTN_OK rather than NULL as a second argument to

Pass ESD_BTN_OK rather than NULL as a second argument to
"simple_dialog()"; NULL might be #defined to be a pointer expression on
some platforms, causing compiler warnings (and, on platforms where a
null pointer doesn't have all its bits 0, possibly causing misbehavior,
although I don't think there are any such platforms on which Ethereal
runs).

Don't allow 0 as button mask argument to "simple_dialog()".

Squelch a compiler warning.

Report fatal problems as errors, not warnings.

Report file I/O errors with "file_open_error_message()".

Report file write errors (including those reported by "close()", e.g.
some errors writing to an NFS server) when saving raw packet data to a
file.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@9915 f5534014-38df-0310-8fa8-9805f1628bb7