Fix building error under mingw

Renamed: global_passwords_file -> global_auth_file, put_delete_passwords_file -> put_delete_auth_file.

Using poll() instead of select() to prevent FD_SETSIZE overflows, for programs that open large number of descriptors

In mg_upload(), use fopen() with "wb" mode instead of "wbx", cause "x" is not understood by Mac and Windows.