Set FD_CLOEXEC flag on all sockets.

Set FD_CLOEXEC flag on all sockets.

Scripts called by tinc would inherit its open filedescriptors. This could
be a problem if other long-running daemons are started from those scripts,
if those daemons would not close all filedescriptors before going into the
background.

Problem found and solution suggested by Nick Hibma.