Fix systemd main pid handling when running with --log

[brandond]

Fixes regression introduced in #4115, reported by @kralicky

Proposed Changes

Using MAINPID breaks systemd's exit detection, as it stops watching the
original pid, but is unable to watch the new pid as it is not a child
of systemd itself. The best we can do is just notify when execing the child
process.

We also need to consolidate forking into a single place so that we don't
end up with multiple levels of child processes if both redirecting log
output and reaping child processes.

Types of Changes

bugfix

Verification

• start k3s in docker without the docker --init flag; note that it does not leave behind zombie containerd-shim processes.
• start k3s as systemd service with the --log flag; note that logs are written to the specified log file.
• start k3s as systemd service with the --log flag; note that it is restarted properly when killed.

Linked Issues

• Sending MAINPID to systemd causes systemd to not restart k3s when started with --log #4189

User-Facing Change

Fixed an issue that caused k3s to not be restarted by systemd when started with the `--log` flag.

Further Comments

[codecov-commenter]

Codecov Report

Merging #4190 (ac3759f) into master (ac7a8d8) will increase coverage by 0.00%.
The diff coverage is 0.00%.

@@           Coverage Diff           @@
##           master    #4190   +/-   ##
=======================================
  Coverage   11.46%   11.47%           
=======================================
  Files         134      135    +1     
  Lines        9242     9237    -5     
=======================================
  Hits         1060     1060           
+ Misses       7956     7951    -5     
  Partials      226      226           

Flag	Coverage Δ
inttests	0.67% <0.00%> (+<0.01%)	⬆️
unittests	11.12% <0.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/cli/agent/agent.go	0.00% <0.00%> (ø)
pkg/cli/cmds/init_linux.go	0.00% <0.00%> (ø)
pkg/cli/cmds/log.go	0.00% <0.00%> (ø)
pkg/cli/cmds/log_linux.go	0.00% <0.00%> (ø)
pkg/cli/server/server.go	0.00% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update ac7a8d8...ac3759f. Read the comment docs.