When dialing with a relative Timeout instead of an absolute Deadline,
the deadline function only makes sense if called before doing any
time-consuming work.

This change calls deadline exactly once, storing the result until the
Dial operation completes.  The partialDeadline implementation is
reverted to the following patch set 3:
https://go-review.googlesource.com/#/c/8768/3..4/src/net/dial.go

Otherwise, when dialing a name with multiple IP addresses, or when DNS
is slow, the recomputed deadline causes the total Timeout to exceed that
requested by the user.

Fixes #11796

Change-Id: I5e1f0d545f9e86a4e0e2ac31a9bd108849cf0fdf
Reviewed-on: https://go-review.googlesource.com/12442
Run-TryBot: Paul Marks <pmarks@google.com>
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>