libhttpserver 2.0.0

v2.0 release. See RELEASE_NOTES.md for the full v1->v2 porting summary.
SOVERSION bumped: shared library now ships as libhttpserver.so.2
	(Linux) / libhttpserver.2.dylib (Darwin). Parallel-installable
	with any prior libhttpserver.so.0/1 on disk (runtime artefacts
	only; dev-time .la/.a/.pc/headers are last-installer-wins by
	design).
Added per-function cyclomatic complexity gate
	(scripts/check-complexity.sh via `lizard`) and copy/paste
	detection gate (scripts/check-duplication.sh via PMD CPD with
	the C++ tokenizer), wired into the existing CI lint lane and
	exposed locally as `make lint-complexity` /
	`make lint-duplication`. Initial CCN ceiling intentionally set
	above the current worst offender so CI stays green; ratcheted
	down per refactor commit until reaching the long-term target
	of 10. Duplication gate runs at PMD's default
	minimum-tokens=100 from day one (currently zero hits).

Version 0.19.0

    Considering family_url as part of the priority when selecting a URL to match.
    More explicit selection of C++ version.
    Ability to handle multiple parameters with the same name on the URL.
    Enable error_log to handle params to interpolate in the string.
    Reduced memory footprint of requests using of string_view.
    Configurable management of upload files that allows to load files on disk only, memory only or both disk and memory.
    Fixed error codes.
    Method not allowed now returns 'allows' header.
    Added method to access the gnutls_session struct.
    Code cleanups.
    Better use of RAII.
    Improved test coverage.