curses: change from malloc() to nethack's alloc()

curses: change from malloc() to nethack's alloc()

There was no provision for malloc() potentially returning Null and it
wasn't integrated with nethack's MONITOR_HEAP.  'heaputil' shows that
the curses interface is leaking like a sieve.  If some things are
actually being allocated separately and then freed from within curses,
those need to be thoroughly documented and maybe switched back to
malloc().