time-util: make parse_timestamp() use the RFC-822/ISO 8601 standard timezone spec

time-util: make parse_timestamp() use the RFC-822/ISO 8601 standard timezone spec

If the timezone is specified with a number e.g. +0900 or so, then
let's parse the time as a UTC, and adjust it with the specified time
shift.

Otherwise, if an area has timezone change, e.g.
---
Africa/Casablanca  Sun Jun 17 01:59:59 2018 UT = Sun Jun 17 01:59:59 2018 +00 isdst=0 gmtoff=0
Africa/Casablanca  Sun Jun 17 02:00:00 2018 UT = Sun Jun 17 03:00:00 2018 +01 isdst=1 gmtoff=3600
Africa/Casablanca  Sun Oct 28 01:59:59 2018 UT = Sun Oct 28 02:59:59 2018 +01 isdst=1 gmtoff=3600
Africa/Casablanca  Sun Oct 28 02:00:00 2018 UT = Sun Oct 28 03:00:00 2018 +01 isdst=0 gmtoff=3600
---
then we could not determine isdst from the timezone (+01 in the above)
and mktime() will provide wrong results.

Fixes #26370.