secnet: Provide `load-private' verb

secnet: Provide `load-private' verb

This allows the config file to specify the use of the scheme loadpriv
call with an arbitrary private key file and algorithm, without having
to enable the private key negotiation system.

This is the private key counterpart to
  secnet: Provide `make-public' verb

Like the first provision of `make-public', there is a bug: we use
system_log for reporting errors, but that is not set up until after
the configuration is read.  So errors turn into segfaults.  We will
fix that in a moment.

Aside from that, with this change, new public key schemes can just
provide an entry in the sigschemes table.  They do not need to (and
should not) define their own toplevel config bindings.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>