Introduce pipex_iterator(), the special thing to perform

Introduce pipex_iterator(), the special thing to perform
`pipex_session_list' foreach walkthrough with `pipex_list_mtx' mutex(9)
relocking. It inserts special item after acquired `session' and keeps it
linked until `session' release. Only owner can unlink it's own item, so
the LIST_NEXT(session) is always valid even the `session' was unlinked.
The iterator skips special items at the `session' acquisition time, as
all other foreach loops where `pipex_list_mtx' mutex(9) is not relocked.

ok yasuoka