Only launch transport proxies that provide useful transports.
Simplify transport_is_needed.

By Roger at
https://trac.torproject.org/projects/tor/ticket/5018#comment:11.
Document that unneeded transports are ignored.

Suggested by Roger in
https://trac.torproject.org/projects/tor/ticket/5018#comment:11.
Merge remote-tracking branch 'public/bug5018'

Conflicts:
	src/or/entrynodes.c