Use utmpx on FreeBSD >= 9.0.

In FreeBSD 9 `utmp.h` has been replaced by `utmph.x` and because of this, sigar doesn't compile on FreeBSD 9.

This patch fixes this and makes it use utmpx.h if FreeBSD version is >= 9 and it also sets the correct path for the utmpx file[1][2].

1: http://lists.freebsd.org/pipermail/freebsd-current/2010-January/014893.html
2: http://en.wikipedia.org/wiki/Utmp