Track the last read and last write timestamp at the device level and modify

Track the last read and last write timestamp at the device level and modify
the stat code to retrieve the information.  This is so devices such as ttys
report the correct access/modified time for the 'w' and related utilities.
NOTE: the inode still needs to be updated at CLOSE time to record the last
accessed and modified times persistently, and this is not yet occuring.

This is necessary because device read/write now bypasses the filesystem VOP
code.