Use ui_out for "info checkpoints"

Use ui_out for "info checkpoints"

In his review of my recent checkpoint work (commit e5501dd4321),
Andrew Burgess suggested that I use GDB's structured table generation
mechanism for the "info checkpoints" command.  This patch does
that.

Andrew also recommended using print_stack_frame() for the "Frame"
column.  I tried this, but ran into some problems, which are described
in a comment in the code.  I got it to mostly work, except for the
case when the current/active fork is running.  Switching context away
from and then back to a running fork doesn't work.  It could, perhaps,
be made to work, but I'm not convinced that the checkpoint facility is
important enough to expend the effort for this case.  So, instead,
I simply adapted the existing checkpoint frame printing code to
use the ui_out machinery instead of gdb_printf.

Approved-By: Tom Tromey <tom@tromey.com>