Calculate midnight (for volume stats) based on standard functions

Calculate midnight (for volume stats) based on standard functions

The "tz" structure used with gettimeofday is not blessed by common
standards and not widely supported, on Solaris for example it does not
return anything useful.
Calculate midnight based on standard functions (localtime/mktime).

Change-Id: I516d732d4a9dd4696a524a271d806d349c074c25
Reviewed-on: http://gerrit.openafs.org/849
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>