Allow overriding the location of Samba's smbd.

Pretty much every OS I look at has some means of
changing this path (patching) so lets just make
it easier for OS developers creating packages
and/or end users to override the location.

Signed-off-by: Brad Smith <brad@comstyle.com>
Reviewed-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>