Merge of DSA mod inverse fix to 1.0.2. - #7512

There is a side channel attack against the division used to calculate one of
the modulo inverses in the DSA algorithm. This change takes advantage of the
primality of the modulo and Fermat's little theorem to calculate the inverse
without leaking information.

Thanks to Samuel Weiser for finding and reporting this.

Reviewed-by: Richard Levitte <[email protected]>
(Merged from #7512)

Low severity timing vulnerability in the DSA signature algorithm

Publicly disclosed but unreleased, pending OpenSSL 1.0.2q, not deemed
severe enough to be assigned a CVE #.

Ref: openssl/openssl#7487
Ref: openssl/openssl#7512
Ref: nodejs#23965
Upstream: openssl/openssl@415c3356
Upstream: openssl/openssl@ebf65dbe

Original commit message:

    DSA mod inverse fix

    There is a side channel attack against the division used to calculate one of
    the modulo inverses in the DSA algorithm.  This change takes advantage of the
    primality of the modulo and Fermat's little theorem to calculate the inverse
    without leaking information.

    Thanks to Samuel Weiser for finding and reporting this.

    Reviewed-by: Matthias St. Pierre <[email protected]>
    Reviewed-by: Bernd Edlinger <[email protected]>
    (Merged from openssl/openssl#7487)

Original backport commit message:

    Reviewed-by: Richard Levitte <[email protected]>
    (Merged from openssl/openssl#7512)