sd-event: add pidfd support

      This adds support for watching for process exits via Linux new pidfd
concept. This makes watching processes and killing them race-free if
properly used, fixing a long-standing UNIX misdesign.

This patch adds implicit and explicit pidfd support to sd-event: if a
process shall be watched and is specified by PID we will now internally
create a pidfd for it and use that, if available. Alternatively a new
constructor for child process event sources is added that takes pidfds
as input.

Besides mere watching of child processes via pidfd two additional
features are added:

→ sd_event_source_send_child_signal() allows sending a signal to the
  process being watched in the safest way possible (wrapping
  the new pidfd_send_signal() syscall).

→ sd_event_source_set_child_process_own() allows marking a process
  watched for destruction as soon as the event source is freed. This is
  currently implemented in userspace, but hopefully will become a kernel
  feature event