Use libssh_rc instead of libssh.

Stef needs 0.6 for the GSSAPI work, I believe.  I have uploaded libssh 0.6-rc2 to cockpit-deps which seems to be new enough for libgssh.

With this, VERIFY almost passes: only check-multi-machine fails.  More about this soon...

(I can't say I understand exactly what happens when mixing shared and static library like we do, but the result seems to work.  Once libssh 0.6 is out and in the distro, we should switch to it, of course.)