Use the gettime module in ls.c.

Use the gettime module in ls.c.

Implement TODO list item to make ls.c use the gettime module.
* TODO (ls): Now that we use gettime, remove the TODO entry.
* src/ls.c: Use timespec.h and the gettime module.
(current_time): Change type from time_t to struct timespec.
(current_time_ns): Removed.
(get_current_time): Removed.
(print_long_format): Remove when and when_ns, since we have
when_timespec anyway.  Change type of variable six_months_ago from
time_t to struct timespec.