various fixes:

  use linked list macros for managing backend list (inspired by bug #4258)
  use ast_copy_string instead of strncpy when appropriate
  minor fixes and formatting cleanup
  add AST_LIST_HEAD_STATIC and AST_LIST_REMOVE_CURRENT macros


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@5659 65c4cc65-6c06-0410-ace0-fbb531ad65f3