* libc/include/time.h (__tzrule_struct): Make offset long, since
a 16-bit int overflows on a 12-hour offset.
* libc/sys/linux/include/time.h: Ditto.
* libc/time/mktime.c (mktime): Use new type of __tzrule.offset.
* libc/time/mktm_r.c: Ditto.
* libc/time/gettzinfo.c: Ditto.
* libc/time/strftime.c (strftime): Fix '%x' to deal with negative
years. Fix '%z' to use long, not int.