autocomplete fix for win32

autocomplete fix for win32

The problem with the new autocomplete was tracked down to
be the result of differences between different implementations
of backsp().  The differences go all the way back to the
early MSDOS port by the look of it, and the win32,
and Mac tty ports all seemed to pattern themselves after the
MSDOS port for that routine.  Apparently, it didn't cause any
harm until now.

The problem is that backsp() sends a character sequence
of 0x08, 0x20, 0x08 on at least those ports, where the Unix
tty code only sends 0x08.  So the characters in the new
autocomplete were all being erased from the screen.

This patch only fixes the win32 tty port, so I've left the
conditional code in getline.c for DOS and Mac. I