leapdb: support leap-seconds.list as second source

leapdb: support leap-seconds.list as second source

The existing implementation of getting leap second information from a
timezone in get_tz_leap() relies on non-portable C library behaviour.

Specifically, mktime is not required to return '60' in the tm_sec field
when a leap second is inserted leading to "Timezone right/UTC failed
leap second check, ignoring" errors on musl based systems.

This patch adds support for getting leap second information from the
leap-seconds.list file included with tzdata and adds a new configuration
directive leapseclist to switch on the feature.