Fix process-attributes time precision on GNU/Linux

Fix process-attributes time precision on GNU/Linux

* src/sysdep.c [GNU_LINUX]:
(time_from_jiffies): Simplify by using time-convert.  Change args
and result type.  All uses changed.
(ltime_from_jiffies): Remove; call time_from_jiffies instead.
(put_jiffies): New function.
(get_up_time): Return Lisp_Object not struct timespec.
All uses changed.  Simplify by using time-add.
(system_process_attributes): Simplify by using the above.
This fixes some minor problems where timestamps promised more
precision than was actually available.  When info is not
available (e.g., sysconf fails) do not place it into the alist.
* src/timefns.c (float_time): Now extern.