[SSSD] [sssd PR#964][opened] util/watchdog: fixed watchdog implementation

[alexey-tikhonov]

   URL: https://github.com/SSSD/sssd/pull/964
Author: alexey-tikhonov
 Title: #964: util/watchdog: fixed watchdog implementation
Action: opened

PR body:
"""
In case watchdog detected locked process and this process was parent
process it just sent SIGTERM to the whole group of processes, including
itself.
This handling was wrong: generic `server_setup()` installs custom
libtevent handler for SIGTERM signal so this signal is only processed
in the context of tevent mainloop. But if tevent mainloop is stuck
(exactly the case that triggers WD) then event is not processed
and this made watchdog useless.
`watchdog_handler()` and `watchdog_detect_timeshift()` were amended to do
unconditional `_exit()` after optionally sending a signal to the group.

Resolves: https://pagure.io/SSSD/sssd/issue/4089
"""

To pull the PR as Git branch:
git remote add ghsssd https://github.com/SSSD/sssd
git fetch ghsssd pull/964/head:pr964
git checkout pr964