This fixes up the *BSD handling within the Operating System check.
SYS is now set to the OS without the version attached as provided
by ${host_os} (.e.g. openbsd instead of openbsd5.1) to match all
the other OS's and simplifies other OS's checks further down. This
also allows the script to recognize DragonFly.
Signed-off-by: Rémi Denis-Courmont <remi@remlab.net>