http.c: allow custom TCP keepalive behavior via config

curl supports a few options to control when and how often it should
instruct the OS to send TCP keepalives, like KEEPIDLE, KEEPINTVL, and
KEEPCNT. Until this point, there hasn't been a way for users to change
what values are used for these options, forcing them to rely on curl's
defaults.

But we do unconditionally enable TCP keepalives without giving users an
ability to tweak any fine-grained parameters. Ordinarily this isn't a
problem, particularly for users that have fast-enough connections,
and/or are talking to a server that has generous or nonexistent
thresholds for killing a connection it hasn't heard from in a while.

But it can present a problem when one or both of those assumptions fail.
For instance, I can reliably get an in-progress clone to be killed from
the remote end when cloning from some forges while using trickle to
limit my clone's bandwidth.

For those users and others who wish to more finely tune the OS's
keepalive behavior, expose configuration and environment variables which
allow setting curl's KEEPIDLE, KEEPINTVL, and KEEPCNT options.

Note that while KEEPIDLE and KEEPINTVL were added in curl 7.25.0,
KEEPCNT was added much more recently in curl 8.9.0. Per f7c094060c
(git-curl-compat: remove check for curl 7.25.0, 2024-10-23), both
KEEPIDLE and KEEPINTVL are set unconditionally. But since we may be
compiled with a curl that isn't as new as 8.9.0, only set KEEPCNT when
we have CURLOPT_TCP_KEEPCNT to begin with.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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