Use bignum static constants instead of BN_bin2bn()

[huangqinjin]

keep consistent with bn_dh.c bn_nist.c bn_srp.c

[richsalz]

rather than cut/paste and duplicate the bn_pack macros, they should be put into one place in bn_lcl.h and used everywhere consistently.

[mattcaswell]

Why #if 0 the old code out rather than just deleting it?

[huangqinjin]

@richsalz bn_srp.c defined the bn_pack4 macro, but I think bn_pack2 marco should work well. How to make them consistent and then put into bn_lcl.h.This is my first try to contribute to openssl, so any word helps me. Thank you.

[richsalz]

Post to openssl-dev; good questions.