configure finds mtx also at /usr/sbin/mtx

some distribution store mtx under /usr/sbin/mtx.
As configure runs as normal user, this path is not set.
This patch let configure also look at sbin paths.

fixes #101: variable MTX is not defined

Signed-off-by: Marco van Wieringen <marco.van.wieringen@bareos.com>
added check for mtx program

Signed-off-by: Marco van Wieringen <marco.van.wieringen@bareos.com>
Rebuild configure.