add backtrace in unix_signal_handler

add backtrace in unix_signal_handler

crash stack backtrace will be directed to syslog

1. make use of glic backtrace in execinfo.h. the old clib_backtrace is removed
2. install SIGABRT in signal handler, but have to remove it when backtrace is
   done. reason is to capture stack trace caused by SIGABRT. vPP ASSERT always
   call os_exit then abort().  we definitely want to know the trace of this
   situation. It is a little tricky to avoid SIGABRT infinite loop
3. always load symbols by calling  clib_elf_main_init () in main(). Otherwise,
   PC addresses instead of symbols will be displayed.

Change-Id: I150e15b94a4620b2ea4f08c73dc3e6ad1856de1e
Signed-off-by: Kingwel Xie <kingwel.xie@ericsson.com>