Improve EVP_PKEY_sign documentation

Clarify the intended use of EVP_PKEY_sign. Make the code example compile.

Reviewed-by: Dr Stephen Henson <steve@openssl.org>
RT3061: Don't SEGFAULT when trying to export a public DSA key as a private key.

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

From upstream commit e19c938.
RT3060: Limit the number of empty records.
Limit the number of empty records that will be processed consecutively
in order to prevent ssl3_get_record from never returning.

Reported by "oftc_must_be_destroyed" and George Kadianakis.

Reviewed-by: Bodo Moeller <bodo@openssl.org>

From upstream commit 3aac17a.