secnet: Provide `make-public' verb

secnet: Provide `make-public' verb

This allows the config file to specify the use of the scheme loadpub
call without having to put public keys in separate files.  That will
be useful for testing and perhaps in installations that just want
fixed keys.

There is a bug here: 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.

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