gpg: --gen-random code cleanup by using es_set_binary.

* g10/gpg.c (main): Replace setmode by es_set_binary and use only when
needed.

It is better to use our es_set_binary than to use a Windows specific
method which still worked but is fragile because estream might be
changed. We now set binary only when needed. Note that it does not
harm to call es_set_binary more often than needed.