smbd: Move deadtime default to parameter definition and man page

smbd: Move deadtime default to parameter definition and man page

The code has a default of one week (10080 minutes) if the parameter is
set to 0. Make this the public default of the parameter, instead of
hiding it in the code. This change also has the code match the
documentation that setting this parameter to 0 disables the check.

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>