timezonemap command

Generator for timezonemap.go. Fetches Firebird's TimeZones.h and emits both switch functions plus getTimezoneVersion.

Usage:

go run ./_attic/timezonemap                          # fetch from upstream master
go run ./_attic/timezonemap -url <url>               # custom branch/tag
go run ./_attic/timezonemap -src /path/to/TimeZones.h # local file
go run ./_attic/timezonemap -out /path/to/timezonemap.go