Add code in dlg_filesel_set and dlg_fontsel_set which makes them

Add code in dlg_filesel_set and dlg_fontsel_set which makes them
duplicate the strings they pass to gtk_entry_set_text. I was already
doing that in dlg_editbox_set, but forgot to add the same code when I
revamped FontSpec and Filename to contain dynamically allocated
strings (r9314 and r9316 respectively). This fixes a bug where, on
some versions of GTK (but apparently not up-to-date versions), loading
a saved session causes gibberish to appear in file-selector edit boxes
accompanied by a valgrind error.

git-svn-id: http://svn.tartarus.org/sgt/putty@9456 cda61777-01e9-0310-a592-d414129be87e