Change set_system_time(struct tm *) to set_system_time(time_t)

git-svn-id: http://svn.savannah.nongnu.org/svn/avr-libc/trunk@2356 1084037a-409f-4a99-8160-1a4b3577d4c1
Add file libc/time/print_lz.c, an auxillary function shared between isotime_r and asctime_r.
asctime now avoids sprintf() and all that entails.

git-svn-id: http://svn.savannah.nongnu.org/svn/avr-libc/trunk@2357 1084037a-409f-4a99-8160-1a4b3577d4c1