Add systemd socket activation support

[drizzt]

Problem

Restarting valkey-server closes its listening sockets, so clients that
connect during the restart window see ECONNREFUSED instead of a brief
delay. With systemd socket activation, systemd owns the listening fds
and holds them open across service restarts, so in-flight connections
simply wait in the kernel backlog while the new process starts up.

Fix

When started under a systemd .socket unit, valkey inherits the
listening fds via LISTEN_FDS and skips its own bind()/listen()
for those listeners.

Detection is automatic. After config load, sd_listen_fds() is
consulted and each inherited fd is matched to a configured listener by
probing:

• AF_UNIX fds are matched against server.unixsocket by sun_path.
• AF_INET/AF_INET6 fds have their local address and port extracted via
  anetFdToString() and matched against server.port, server.tls_port,
  or the cluster bus port (server.cluster_port, falling back to
  server.port + CLUSTER_PORT_INCR), and against server.bindaddr[]
  with the Valkey wildcards * (any IPv4) and ::* (any IPv6).

An fd whose address/port does not match any configured listener is
closed with a warning, so a .socket unit that exposes a broader
address than valkey.conf authorizes will not silently widen exposure.

Listeners with no matching inherited fd fall through to normal
bind()/listen(), so partial activation (e.g. unix-only) works. An
inherited unix socket is never unlinked on shutdown since systemd owns
the path.

Runtime reconfiguration via CONFIG SET port / bind / tls-port is
refused on an inherited listener: closing and re-binding would
relinquish the fd that systemd still tracks and the subsequent bind()
can fail or race. A clear log message asks the operator to adjust the
.socket unit instead.

Build integration

• Make: no change; USE_SYSTEMD already detects libsystemd and
  defines HAVE_LIBSYSTEMD.
• CMake: add matching pkg-config detection of libsystemd, enabling
  HAVE_LIBSYSTEMD and sd_notify parity with the Make build.
  USE_SYSTEMD=yes is honored as a hard requirement (fatal if
  pkg-config or libsystemd is missing), matching the Make build.

The sample utils/systemd-valkey_server.service is documented with
usage instructions and a companion utils/systemd-valkey_server.socket
unit is provided.

Testing

New tests in tests/unit/socket-activation.tcl use systemd-socket-activate
(systemd >= 258, with --now) to simulate activation and verify:

• TCP and unix inherited fds are adopted.
• A mismatched bind address is rejected rather than adopted.
• The unix socket path is preserved on shutdown.
• CONFIG SET port is refused on an inherited listener.
• Self-binding is unaffected when no fds are inherited.

The tests skip cleanly when systemd-socket-activate or libsystemd is
not available.