crypto/tls: add ALPN support

[mikioh]

See http://tools.ietf.org/html/draft-ietf-tls-applayerprotoneg.

I'm fine either with or without client hello padding (or blowing up balloons) for some
intermediaries. I think that's agl's pick.
http://www.ietf.org/mail-archive/web/tls/current/msg10423.html

[rsc]

Comment 1:

Labels changed: added go1.3maybe.

[rsc]

Comment 2:

Labels changed: added release-none, removed go1.3maybe.

[rsc]

Comment 3:

Labels changed: added repo-main.

[bradfitz]

Comment 4:

Adam, is this feasible (by you or somebody else) before Go 1.4, for HTTP/2 support?

Owner changed to @agl.

Status changed to Accepted.

[mikioh]

Comment 5:

Just published: http://tools.ietf.org/html/rfc7301

[gopherbot]

Comment 6:

CL https://golang.org/cl/108710046 mentions this issue.

[agl]

Comment 7:

This issue was closed by revision d0e255f.

Status changed to Fixed.