Change initialization code to allocate a minimum of 256 file handles

Change initialization code to allocate a minimum of 256 file handles
for Scheme.  The old default of 20 is too small -- quickly eaten by a
small number of subprocesses in Edwin -- and worse, the default is not
a constant but depends on the method used to invoke Scheme.

The number of file handles can by dynamically changed using a new
primitive.  The channel table is initialized to 1024 entries, allowing
plenty of room for growth should that be desired.