ipa-pwd-extop: use timegm() instead of mktime() to preserve timezone offset
    
    "Kerberos principal expiration" is set in UTC and when server is in
    different timezone, the time difference between timezone is respected by
    the IPA server/client for Kerberos authentication.
    
    The problem is due to mktime() assuming default time zone but since we
    parse the time using Zulu (UTC+0) timezone, mktime() forces current time
    zone offset added.
    
    The method is using mktime() and comparing to the current time obtained
    with time(NULL). According to its man page, mktime is considering the
    time as local time:
    
       The mktime() function converts a broken-down time structure,  expressed
       as  local  time, to calendar time representation.
    
    Instead mktime() we should use timegm(). The problem is that it is
    non-standard GNU extension and it is recommended (in the man page for
    timegm(3)) to avoid its use. An alternative is to set TZ=UTC, call
    mktime(), unset TZ, but since we are running in a multi-threaded
    environment this is problematic.
    
    On the other hand, we already rely on GNU extensions and enable them
    with -D_DEFAULT_SOURCE=1, so use of timegm() is enabled already.
    
    The fix, therefore, is to use timegm() instead of mktime() in
    daemons/ipa-slapi-plugins/ipa-pwd-extop/prepost.c in two places where we
    first do 'strptime()' with Zulu time zone (in ipapwd_pre_bind() and
    ipapwd_write_krb_keys()).
    
    Fixes: https://pagure.io/freeipa/issue/8362
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-by: Simo Sorce <simo@redhat.com>
    Reviewed-By: Simo Sorce <ssorce@redhat.com>
    Reviewed-By: Christian Heimes <cheimes@redhat.com>