inprocess: set Main instance fields from Main._init.

The Object clone process borrowed from lua-stdlib only makes a
shallow copy of the prototype's fields when instantiating a new
object.  This means that any table references in the prototype
end up being shared by the cloned object.
* lib/specl/main.lua: ...unless we always set new values in the
_init method.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>