handle_segfault is the signal handler code of mysqld.  however, it makes
calls to potentially unsafe functions localtime_r, fprintf, fflush.



include/my_stacktrace.h:
  Add safe versions of itoa() write() and snprintf().
libmysqld/CMakeLists.txt:
  Move signal handler to separate file.
mysys/stacktrace.c:
  Remove unsafe function calls.
sql/CMakeLists.txt:
  Move signal handler to separate file.
sql/mysqld.cc:
  Move signal handler to separate file.
sql/set_var.h:
  Add missing #include dependency.
sql/sys_vars.cc:
  Cleanup .h and .cc files.
sql/sys_vars.h:
  Cleanup .h and .cc files.