MINOR: server: add 'clear counters server <backend>/<server>' CLI command

MINOR: server: add 'clear counters server <backend>/<server>' CLI command

Add a CLI command to reset the statistics counters of a single server:

  clear counters server <backend>/<server> [force]

Motivation: 'clear counters all' resets counters across every proxy and
server in the process, which is too blunt for common operational needs.
In particular, when a server slot is being reused to represent a
different logical entity (e.g. a different Kubernetes pod occupying the
same slot after 'set server <b>/<s> name'), the operator needs per-slot
counter attribution and cannot afford to wipe the entire process' stats.

The command requires ACCESS_LVL_ADMIN, like 'clear counters all'. It
resets cumulative counters (not just the max gauges cleared by the base
'clear counters'), so restricting it to admin prevents an operator-level
socket from hiding accumulated activity. Like 'clear counters' / 'clear
counters all', it is not gated by the server's administrative state: the
reset only zeroes counter values and does not touch the server object or
its runtime state, so it is safe to issue on a live server (a concurrent
counter increment races on a value exactly as it already does for 'clear
counters all').

When the server's counters are registered in a shared-memory stats file
object (COUNTERS_SHARED_F_LOCAL not set), clearing them breaks the
monotonicity that monitoring tools consuming the shared stats rely on
and affects every process attached to the object. Such a clear is
therefore refused unless the optional 'force' argument is given.

The reset covers both the native server counters, via counters_be_reset()
introduced earlier in this series (which safely zeroes the per-tgroup
accumulated counter contents while preserving the shared.tg pointer
array), and the module-registered extra counters, via the new
srv_stats_clear_extra_counters() helper.

Note on the dispatch: 'clear counters' is a two-word CLI keyword, and
cli_find_kw() matches the first keyword whose tokens are all consumed,
so a separate three-word 'clear counters server' keyword would be
shadowed (or would shadow) depending on registration order. To avoid
that ambiguity the new sub-command is dispatched from within
cli_parse_clear_counters() when args[2] == "server", mirroring how the
existing "all" argument is handled. The per-server worker lives in
server.c (cli_clear_counters_server()) where the server lookup and
lock helpers are available.