- Add support for "paging" in stack trace output. That is, when you do

- Add support for "paging" in stack trace output.  That is, when you do
  a stack trace from ddb, the output will pause with a '--More--' prompt
  every 18 lines.  If you hit Enter, it will print another line and prompt
  again.  If you hit space it will output another page and then prompt.
  If you hit 'q' or 'x' it will abort the rest of the stack trace.
- Fix the sparc64 userland stack trace to honor the total count of lines
  to print.  This is useful if your trace happens to walk back onto
  0xdeadc0de and gets stuck in an endless loop.

MFC after:	1 month
Tested on:	i386, alpha, sparc64