BUG/MINOR: init: do not call fork_poller() for non-forked processes

BUG/MINOR: init: do not call fork_poller() for non-forked processes

In 3.1-dev10, commit 8dd4efe42f ("MAJOR: mworker: move master-worker
fork in init()") made the fork_poller() code unconditional, while it
is only desirable for processes that have been forked from a parent
(standalone daemon mode) or from a master (master-worker mode). The
call can be expensive in some cases as it will create a new poller,
scan and try to migrate to it all existing FDs till the highest known
one. With very high numbers of FDs, this can take several seconds to
start.

This should be backported to 3.1.