dispatch: Fix several connect-related issues

dispatch: Fix several connect-related issues

- startrecv() and getnext() have been rewritten.

- Don't set TCP flag when connecting a UDP dispatch.

- Prevent TCP connections from trying to connect twice.

- dns_dispatch_gettcp() can now find a matching TCP dispatch that has
  not yet fully connected, and attach to it.  when the connection is
  completed, the connect callbacks are run for all of the pending
  entries.

- An atomic 'state' variable is now used for connection state instead of
  attributes.

- When dns_dispatch_cancel() is called on a TCP dispatch entry, only
  that one entry is canceled. the dispatch itself should not be shut
  down until there are no dispatch entries left associated with it.

- Other incidental cleanup, including removing DNS_DISPATCHATTR_IPV4 and
  _IPV6 (they were being set in the dispatch attributes but never used),
  cleaning up dns_requestmgr_create(), and renaming dns_dispatch_read()
  to the more descriptive dns_dispatch_resume().