File descriptor cleanup stage 2, remove the separate arrays for file

File descriptor cleanup stage 2, remove the separate arrays for file
pointers, fileflags, and allocation counts and replace the mess with a
single structural array.  Also revamp the code that checks whether the
file descriptor array is built-in or allocated.

Note that the removed malloc's were doing something weird, allocating
'nf * OFILESIZE + 1' bytes instead of 'nf * OFILESIZE' bytes.  I could
not find any reason at all why it was doing that.  It's gone now anyway.