Merge branch 'event_reinit-for-signals-v3'

* event_reinit-for-signals-v3:
  test/regress: cover existing signal callbacks and fork() + event_reinit()
  test/regress: cover signals after fork() + event_reinit()
  test/regress: main/fork: rewrite assertions by just removing event in callback
  event_reinit: make signals works after fork() without evsig_add()
  event_reinit: always re-init signal's socketpair

Fixes #307