A 32-bit time_t value overflows when converted to the Y1900 epoch used
in the leapsec list. Use a 64-bit variable in get_list_leap() to fix the
comparisons on systems using 32-bit time_t.
Fixes: 53823b9f1c ("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.
