cmake: add support

[vszakats]

It supports building the manual, completions, run tests and install all
these.

Options:

• TRURL_EMABLE_MANUAL: Build the trurl man page (requires Perl). Default: ON
• TRURL_COMPLETION_ZSH: Install zsh completions (requires POSIX shell). Default: OFF
• TRURL_TESTS: Run tests (requires Python). Default: ON
  Test targets: trurl-test, trurl-test-memory.
• TURLS_DISABLE_INSTALL: Disable installation targets. Default OFF
• TRURL_WERROR: Turn compiler warnings into errors. Default: OFF
• CURL_INCLUDE_DIR: Absolute path to curl include directory.
• CURL_LIBRARY: Absolute path to libcurl library.
• CURL_USE_STATIC_LIBS: Look for static libcurl library (requires CMake v3.28).

Also:

• CI: test CMake builds on Linux, macOS, Cygwin, Windows (MSVC). Windows
  uses the latest curl-for-win binary from curl.se/windows.
• trurl: disable punycode feature for Windows. It doesn't pass the 4
  tests requiring this feature, as tested with both Unicode and non-Unicode
  libcurl builds.
• trurl: fix dangling variable (reported by MSVC).

  trurl.c(1206,1): warning C4701: potentially uninitialized local variable 'olen' used
  

• trurl: fix unreachable code (reported by MSVC).

  trurl.c(830,1): warning C4702: unreachable code
  

• trurl: disable MSVC CRT warnings on insecure fopen() and
  strerror().
• include cmake documentation in CMakeLists.txt itself.
• test.py: show filename of trurl binary.
• generate_completions.sh: replace hardcoded output with stdout.
  To not pollute the source tree in out-of-tree builds.
• generate_completions.sh: convert to POSIX shell.
• generate_completions.sh: set -eu.
• generate_completions.sh: fix indent.
• merge completions directory into scripts.
• .gitignore: not building kernel modules.
• CI: add cmake linter.
• CI: make Cygwin build steps "hermetic", by excluding everything from
  PATH, except Cygwin.
• CI: drop a redundant Cygwin package.
• CI: dedupe checksrc into its own job.
  Also to untie it from Makefile.

---

• CI tests.

[vszakats]

4 tests fail on Windows. I presume it's due to missing Unicode support in trurl and/or libcurl.

All for tests with 'required: "punycode"':
https://github.com/curl/trurl/actions/runs/18636413448/job/53128179515?pr=400

When running the first test with wine, test.py crashes with:

  File "trurl/test.py", line 103, in runCommand
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe4 in position 9: invalid continuation byte

Oh, this was with a libcurl-x64.dll picked up by wine from somewhere on the disk, versioned 7.83.1:

trurl version 0.16.1 libcurl/7.83.1 [built-with 8.16.0]
features: get-empty imap-options no-guess-scheme normalize-ipv4 punycode punycode2idn url-strerror white-space zone-id

It fails normally with the matching 8.16.0 dll (latest curl-for-win official):

trurl version 0.16.1 libcurl/8.16.0 [built-with 8.16.0]
features: get-empty imap-options no-guess-scheme normalize-ipv4 punycode punycode2idn url-strerror white-space zone-id uppercase-hex

Same failure happens with a 8.15.0 dll with Unicode enabled (a private curl-for-win build):

trurl version 0.16.1 libcurl/8.15.0 [built-with 8.15.0]
features: get-empty imap-options no-guess-scheme normalize-ipv4 punycode punycode2idn url-strerror white-space zone-id uppercase-hex

This suggests that trurl doesn't support punycode on Windows.