w32: Fix crash due to bad conversions of utf-8 in the clipboard.

* src/gpgmetools.c (dos_to_unix): Remove.  Remove all calls.
(dump_data_to_clipboard): Return an error code instead of calling
exit.
* src/gpaexportclipop.c
(gpa_export_clipboard_operation_complete_export): Print success
message only on success.

The removal of dos_to_unix is possible due to the patch
https://bugzilla.gnome.org/show_bug.cgi?id=649390 which will go into
the next Gtk+ 2.24 version.dos_to_unix was anyway not correct because
it scanned for an embedded nul but it is not guaranteed that it is
always called with one.

The other patch which will be helpful is:
https://bugzilla.gnome.org/show_bug.cgi?id=696232
(g_utf8_to_utf16() is not guaranteed to succeed. Check the error and
return if it failed.)

Gpg4win uses both patches.

• GnuPG-bug-id: T1525