event: replace obsolete `sigignore()` by `signal()`
systemd: don't use SOCK_CLOEXEC

It is unknown on RHEL5. Moreover, we promise to not exec anything in
this function.
build: more changelog drama

We need ChangeLog to be listed in `dist_doc_DATA` or
`doc_DATA`. Therefore, we list it in the former and if we happen to
install from git, we just ship an empty ChangeLog.
packaging: update packaging stuff for release