top: with lib xalloc changes, restored prior memory logic

top: with lib xalloc changes, restored prior memory logic

With the privatization of the library xalloc functions,
this patch restores top's previous internal memory logic.

This puts us back on a 3.3.1 (and earlier) footing wherein
memory allocation errors carry these implications:
  . if top detected, the terminal will be restored
    prior to an abnormal exit and message
  . if library detected, the terminal will become
    corrupted following the message

To my knowledge, neither type of memory error has ever occurred
during the nine years since top was rewritten.  So the issue of
restoring the termios structure is probably moot.