openssl: RSA fixes for openssl-1.1.

openssl: RSA fixes for openssl-1.1.

In openssl-1.1 the RSA structure has been made opaque, causing
compilation of crypt.c to fail because the code accesses ->n and ->e
directly to set the modulus and the public exponent according to the
values read from the public ssh key.

With openssl-1.1 applications are supposed to call RSA_set0_key()
to set n and e. Unfortunately, this function does not exist in
openssl-1.0.2.

This patch adds a configure check which defines HAVE_RSA_SET0_KEY if
RSA_set0_key() is available. In crypt.c we either call the function
or set ->n and ->e directly, depending on whether HAVE_RSA_SET0_KEY
is defined. This results in code which works on both openssl-1.0.2
and openssl-1.1.0.