Fix a problem seen only on nonconforming systems whereby ls.c's

Fix a problem seen only on nonconforming systems whereby ls.c's
use of localtime, and then of gettimeofday would cause trouble:
the localtime call used to initialize rpl_gettimeofday's save
mechanism would clobber ls's current local time information so
that in any long listing the first file would always be listed
with date 1970-01-01.  Analysis by Volker Borchert.

(localtime): Undefine.
(rpl_localtime): New function.