build: support modern toolchains (GCC 14, Boost 1.78+, C++14)

[gburd]

OSv's master branch does not build on current toolchains. Two independent
breakages compound:

• Boost 1.78+ made boost::system header-only, so libboost_system.a is
  no longer produced. The build hard-errors requiring it. Boost 1.87's
  lockfree headers additionally use std::conditional_t, which needs C++14.
• GCC 14 is stricter: core/mmu.cc's four-argument std::lower_bound needs
  an explicit <algorithm> include, and Boost.Asio has removed the
  long-deprecated io_service, resolver::query, mutable_buffers_1, and
  address_v4::from_string APIs.

I verified that master (8d2d4c8) fails to compile in both C++ modes on
GCC 14.3 / Boost 1.87: gnu++11 dies on Boost's std::conditional_t,
gnu++14 dies on core/mmu.cc lower_bound.

Changes

• Default conf_cxx_level to gnu++14 (conf/base.mk, modules/common.gmk).
• Detect libboost_system only if present; never require it. Support both
  nixpkgs (include/) and FHS (usr/include/) Boost layouts.
• Add <algorithm> to core/mmu.cc and core/pagecache.cc.
• Port Boost.Asio call sites to io_context, range-returning
  resolver::resolve, mutable_buffer, and make_address_v4
  (loader.cc, core/dhcp.cc, tools/cpiod, modules/httpserver-api,
  modules/cloud-init, modules/monitoring-agent, tests/*tcp*).
• Replace boost::math::isinf/isnan with std::isinf/isnan.
• Detect OpenSSL 3.x (libssl.so.3) in addition to 1.1, and fall back to
  LD_LIBRARY_PATH when ldconfig is unavailable.

Notes

• This is the first in a series modernizing OSv's build and bringing in
  musl 1.2.1, virtio-blk multi-queue, and OpenZFS 2.4.2. It is a prerequisite
  for the musl PR (musl 1.2.1 cannot build on GCC 14 without these fixes).
• No functional/runtime behavior change; this is purely build/compat.
• Relates to building on modern distros and NixOS, but contains nothing
  Nix-specific (the Nix dev-env lives outside this PR).