Pass a NULL pointer for the publickey parameter of
libssh2_userauth_publickey_fromfile and
libssh2_userauth_hostbased_fromfile functions. In this case, the
functions recompute the public key from the private key file data.
This is work done by Jean-Louis CHARTON
<Jean-Louis.CHARTON@oikialog.com>, then adapted by Mark Smith and
slightly edited further by me Daniel.
WARNING: this does leave the feature NOT WORKING when libssh2 is built
to use libgcrypt instead of OpenSSL simply due to lack of
implementation.