shutdown: unload perilog plugin during shutdown

[garlick]

Problem: when flux is shutting down and canceling running jobs, the epilog can greatly extend the shutdown time.

Unload the perilog plugin as part of the cleanup tasks so that canceled jobs can complete quickly in the shutdown case.

Note: @grondo's good IMHO idea! This was peeled off of #5860 because it seems like it might be a good candidate for a point release, whereas #5860 is too big of a change for that.

[garlick]

Restarted the fedora38 builder after this failure:

2024-04-09T15:34:00.6879695Z make  check-TESTS
2024-04-09T15:34:00.7586099Z                 test_pty.t:  PASS: N=83  PASS=83  FAIL=0 SKIP=0 XPASS=0 XFAIL=0
2024-04-09T15:34:00.9682855Z            test_terminus.t:  SKIP: N=0   PASS=0   FAIL=0 SKIP=0 XPASS=0 XFAIL=0
2024-04-09T15:34:00.9684195Z ERROR: test_terminus.t - missing test plan
2024-04-09T15:34:00.9685258Z ERROR: test_terminus.t - exited with status 139 (terminated by signal 11?)

[grondo]

LGTM!

[garlick]

Thanks! Setting MWP

[codecov]

Codecov Report

Merging #5871 (77cf439) into master (3464b31) will increase coverage by 0.02%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5871      +/-   ##
==========================================
+ Coverage   83.27%   83.30%   +0.02%     
==========================================
  Files         514      514              
  Lines       82756    82756              
==========================================
+ Hits        68915    68936      +21     
+ Misses      13841    13820      -21     

see 14 files with indirect coverage changes