util: improve backtrace feature

util: improve backtrace feature

improve & clean up the code:
  - correct coding convention
  - add backtrace feature to Mac OS X
    : remove #ifdef CONFIG_DARWIN ... #endif
      The 'backtarce' family functions first appeared in Mac OS X 10.5
  - modify the Windows code using the APIs in DbgHelp
  - change debug channel: from debug_ch to new_debug_ch
  - remove some lines like redundant/unclear/unwanted/etc.
  - modify output form

Change-Id: I8b57d996fa9daabf990932fcfb82e45a5e453636
Signed-off-by: Jinhyung Jo <jinhyung.jo@samsung.com>