Use kqueue as the backend of AE on DragonFlyBSD

[panjf2000]

Currently, we use select(2) on DragonFlyBSD while kqueue is available on DragonFlyBSD since FreeBSD 4.1 and DragonFlyBSD was originally forked from FreeBSD 4.8

select(2) is a pretty old technique that has many defects compared to kqueue, we should switch to kqueue on DragonFlyBSD.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 69.87%. Comparing base (8048abb) to head (24060c9).

Additional details and impacted files

@@             Coverage Diff              @@
##           unstable     #450      +/-   ##
============================================
+ Coverage     69.85%   69.87%   +0.02%     
============================================
  Files           109      109              
  Lines         61791    61791              
============================================
+ Hits          43164    43179      +15     
+ Misses        18627    18612      -15     

see 14 files with indirect coverage changes

[panjf2000]

@madolson Any updates?