crypto/bn/bn_nist.c: bring original failing code back for reference.

RT: 3541
Reviewed-by: Emilia Kasper <emilia@openssl.org>
e_os.h: refine inline override logic (to address warnings in debug build).

Reviewed-by: Dr Stephen Henson <steve@openssl.org>
Parse custom extensions after internal extensions.

Reviewed-by: Rich Salz <rsalz@openssl.org>