add more logging to catch "stuck core-backtrace" problem; reduce gdb looping

We saw the case when user had to kill
abrt-action-generate-core-backtrace after 36 minutes:

Feb  6 14:13:44 localhost abrtd: Generating backtrace
Feb  6 14:50:12 localhost abrtd: 'post-create' on '/var/spool/abrt/ccpp-2013-02-06-14:13:40-1154' exited with 143

ps aux showed this:
USER       PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
root      1719  6.3  7.9 221892 60644 ?        S    14:13   1:46 abrt-action-generate-core-backtrace
   note these --^^^  ^^^                                    ^^^^

It wasn't just waiting for something (like gdb), it was using
some CPU (but not 100%).

This change adds debugging which will narrow it down; and makes
get_backtrace() and run_unstrip_n() a bit more robust.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>