Configuration file: new default location; new default value for GlobalInitFile

Under Linux and macOS, the default location for the configuration file is now
dynare/dynare.ini under the configuration directories as defined by the XDG
specification. Under Windows, the default configuration file is now
%APPDATA%\dynare\dynare.ini.

There is now a default value for the global initialization file (GlobalInitFile
option of the configuration file): the global_init.m in the Dynare
configuration directory.