recover

This is a partial go rewrite of the original C version.

Original C version authors:
Gorka Guardiola, paurea@gmail.com
Russ Cox, rsc@swtch.com
Eric Van Hensbergen, ericvh@gmail.com

Original C version references:
/n/sources/contrib/paurea/recover4e.tgz
/n/sources/contrib/rsc/recover/recover.pdf

This go version currently:
- doesn't work on plan9,
- can't do authentication,
- works on windows and on unix where go works,
- works on android (in termux), though not very useful at the moment,
- works both with or without plan9port,
- uses go net.Dial() address format (host:port),
- when using without plan9port, it won't clean up socket file after terminating,
- needs NAMESPACE set on windows,
- it'll default stay in the foreground.

More details:
https://9fans.topicbox.com/groups/9fans/T1d7bf3c39e885267/recover4-go-rewrite