net: enable TCP keepalives by default

This is just the first step in attempting to make all network connection have
timeouts as a "safe default". TCP keepalives only protect against certain
classes of network and host issues (e.g. server/OS crash), but do nothing
against application-level issues (e.g. an application that accepts connections
but then fails to serve requests).

The actual keep-alive duration (15s) is chosen to cause broken connections
to be closed after 2~3 minutes (depending on the OS, see #23549 for details).
We don't make the actual default value part of the public API for a number of
reasons:
- because it's not very useful by itself: as discussed in #23549 the actual
  "timeout" after which the connection is torn down is duration*(KEEPCNT+1),
  and we use the OS-wide value for KEEPCNT because there's currently no way
  to set it from Go.
- because it may change in the future: if users need to rely on a specific
  value they should explicitly set this value instead of relying on the default.

Fixes #23459

Change-Id: I348c03be97588d5001e6de0f377e7a93b51957fd
Reviewed-on: https://go-review.googlesource.com/c/107196


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>