Fix for bug 7771

This fixes all of the problems with getting and setting timestamps values lower then or equal to time_t(0):

• getting time_t values < 0 works, including the values used as sentinels in Samba 0, -1
• setting time_t to values < -1 works
• setting time_t to 0 works
• setting time_t to -1 works

Fixing the last was extremely tricky as this runs through many conversion codepaths and checks for "value is valid" (ie null_time(), null_nttime() and null_timespec()).