filenamefor your public key. If not used, libcurl defaults to $HOME/.ssh/id_dsa.pub if the HOME environment variable is set, and just "id_dsa.pub" in the current directory if HOME is not set.
Nil (or an empty string) is passed, libcurl will pass no public key to
libssh2, which then tries to compute it from the private key. This is known
to work with libssh2 1.4.0+ linked against OpenSSL.