Also, implement a global OPTIONS * handler, like Apache.

Permit sending "*" requests to handlers, but not path-based
(ServeMux) handlers.  That means people can go out of their
way to support SSDP or SIP or whatever, but most users will be
unaffected.

See RFC 2616 Section 5.1.2 (Request-URI)
See RFC 2616 Section 9.2 (OPTIONS)

Fixes #3692

R=rsc
CC=golang-dev
https://golang.org/cl/6868095