http: Fix overlooked agent.globalAgent export

Noticed by @bnoordhuis in https://github.com/joyent/node/pull/5991#discussion_r5575946
https: fix default port

https.get('https://github.com/') should use port 443, not 80.
test: Move test-http-default-port from disabled to simple
http: add agent.maxFreeSockets option
doc: http keepalive, agent options