NETSCAPE_HANG_BUG is a workaround for a browser bug from many years ago
(2000).
It predates DTLS, so certainly has no place in d1_srvr.c.
In s3_srvr.c it forces the ServerDone to appear in the same record as the
CertificateRequest when doing client auth.

BoringSSL have already made the same commit:
79ae85e4f777f94d91b7be19e8a62016cb55b3c5
Reviewed-by: NTim Hudson <tjh@openssl.org>