Fixed failing of lysp_check_date when using DST configurations with savingtimes >=24h

[Troltsch]

Disabled DST by setting tm_isdst to -1 so that mktime() won't adjust for daylight saving time.
This is especially important for custom DST settings.
Without this fix lysp_check_date() fails due to if (tm.tm_mday != tm_.tm_mday)

This bug can be reproduced i.e. by creating a custom DST like this one:

# Rule NAME    FROM    TO      -   IN   ON      AT    SAVE    LETTER/S
Rule   Custom  2020    max     -   Oct  1       0:00  25:00       S
Rule   Custom  2020    max     -   May  1       0:00  0         -

# Zone NAME    GMT_offset  RULES/SAVE  FORMAT   [UNTIL]
Zone  Custom/TZ    1:00   Custom       C%sT

1. Create the custom_timezone.txt
2. Compile the TZ file: zic custom_timezone.txt
3. Copy the custom TZ to /etc/localtime: cp /usr/share/zoneinfo/Custom/TZ /etc/localtime
4. Change the TZ i.e. with export TZ="Custom/TZ"
5. See that date and date -u have different days due to the insane DST configuration.
6. Run the tests and observe all of them failing without the fix.

Also this fix is compatible with the previous issue when using TZ "Europe/Dublin".

[Troltsch]

Regression tests passed successfully, before and after implementing the fix.

I know this PR seems over the top but I actually had a bug in one of my projects because one of the IETF yang models allowed a range of 1 to 1440 minutes for the recurring offset.

[michalvasko]

It does seem unnecessary but the fix should also not cause any issues, so it can be merged.