param: setup more variables in for the temporary loadparm context for the special...

In doing so, the loadparm context definition is now also moved to the loadparm.h header.

This means that the loadparm context is no longer private to the
lib/param code and that the source3 code can now override the values
within it.

Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>