Add printf-style format checking

Add printf-style format checking

Add the AFS_ATTRIBUTE_FORMAT macro which can be used to turn on
printf-like format checking for our va-arg log functions.

Enable this checking for volser's Log(), and fix the fallout from
doing so.

Change-Id: Ic5ab93ba731856f1f90e894d711b0f84298ded12
Reviewed-on: http://gerrit.openafs.org/789
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>