Bug 5208: Part 1: Restart kids killed by SIGKILL (#1035)

Bug 5208: Part 1: Restart kids killed by SIGKILL (#1035)

OOM killer uses SIGKILL. Squid did not restart kids killed by SIGKILL.
Kids are essential Squid components. Essential components should be
revived (by default) because providing a service without an essential
component would violate Squid functionality requirements.

Squid did not revive a kid killed by SIGKILL because we thought that
doing so will interfere with the "squid -k kill" feature that uses that
signal to kill the whole Squid instance. However, that feature does not
actually work[^1] -- the signal is sent to (and kills) the master
process only, the process which PID is in squid.pid file. This change is
orthogonal to fixing "squid -k kill" (a difficult out-of-scope project).

[^1]: Except in the special case of the no-daemon (squid -N) mode.