connection shutdown, cleanup code

[icing]

Move all shutdown related functions to cshutdn.c and give them Curl_cshutdn_* prefix.

Add connection age handling methods to conncache.h, so that other code does not need to mess with details of how connections keep time.

[Copilot]

🟢 Approval recommended

The relocation and API updates are complete and preserve the existing shutdown and connection-timing behavior.

Pull request overview

Centralizes connection shutdown logic and encapsulates connection-age bookkeeping.

Changes:

• Moves shutdown APIs into cshutdn with consistent naming.
• Adds connection timestamp helpers to the connection pool.
• Updates callers, including Telnet timeout handling.

File summaries

File	Description
lib/cshutdn.c	Implements centralized shutdown operations.
lib/cshutdn.h	Declares the renamed shutdown APIs.
lib/cfilters.c	Uses centralized shutdown-state checks.
lib/cfilters.h	Removes the relocated shutdown declaration.
lib/conncache.c	Manages connection timestamps and shutdown calls.
lib/conncache.h	Exposes connection-age helper APIs.
lib/connect.c	Uses the new shutdown and usage APIs.
lib/connect.h	Removes relocated shutdown APIs.
lib/telnet.c	Uses the standard transfer timeout calculation.
lib/transfer.c	Uses centralized shutdown operations.
lib/url.c	Initializes timestamps through the connection pool.
lib/vdns/hostip.c	Uses the connection-age abstraction.
lib/vtls/vtls.c	Uses renamed shutdown timer APIs.

Review details

• Files reviewed: 13/13 changed files
• Comments generated: 0
• Review effort level: Balanced

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