Added support for newsyslog.

[Newsyslog](http://www.freebsd.org/cgi/man.cgi?query=newsyslog&sektion=8) used on FreeBSD and Mac OS X has the option to compress using bzip2.

It also starts it number suffix at 0 instead of at 1 like logrotate.

Added a check for the existing of a .bz2 extension in `syslog-previous-file` and added a configurable start number for
rotation number suffix (`syslog-number-suffix-start`).

With this change I'll be able to get `syslog-mode` do the right thing on my Mac by adding this snippet to my init file:

```lisp
  (when (eq system-type 'darwin)
    (setq syslog-boot-start-regexp "BOOT_TIME")
    (setq syslog-number-suffix-start 0))
```