Change open_tag() to temporarily disable 'cscopetag' option.

If 'cscopetag' is set, the 'tag' command will actually use the 'cstag' command which in turn performs a 'tjump'. Since 'tjump' doesn't support ranges, if there is more than one match, the default tags menu is displayed. To work around this, we temporarily disable using 'cstag', however, in order to restore the option after a selection has been made we have to use 'exec' instead of 'feedkeys', otherwise the script will exit with the options restored before the 'tag' command is actually run.