cmake: make libssh2-config work with all TLS-backends

CMake:

- Find*: set `<modulename>_FOUND` for compatibility when found via
  `pkg-config`. E.g. `MbedTLS_FOUND`.
  `find_package_handle_standard_args()` sets both `<MODULENAME>_FOUND`
  and `<Modulename>_FOUND` when detecting the dependency. Some CMake
  code relies on this and 3rd-party code may rely on it too. Make sure
  to set the latter variant when detecting the dependency via
  `pkg-config`, where we don't call
  `find_package_handle_standard_args()`.

  CMake sets these variable to `TRUE` (not `ON` or `1`). Replicate this
  for compatibility.

- libssh2-config.cmake: inherit default `LIBSSH2_USE_PKGCONFIG`.
  Follow-up to a3aa6b4ca8 #1525

- document variables consumed by `libssh2-config.cmake.in`.

- `libssh2-config.cmake`: fix to link to non-OpenSSL crypto backends.
  This is most likely not how this is supposed to be done, but better
  than failing.
  What's the canonical way to do this, and how OpenSSL and zlib does it
  is yet to be figured out.

- use `ZLIB::ZLIB` to reference zlib.
- use `IN ITEMS` where missed.
- harmonize variable dump output formats.

CMake `find_package` integration tests:

- extend to all crypto backends (was: OpenSSL).
- show libssh2 variables set by `find_package()`.
- stop building examples and tests for the consumed package.
  For performance.
- enable zlib, for coverage.
- be verbose when building the test targets.

ci/GHA:

- add packaged mbedTLS (2.x) build to Linux matrix.
- alphasort some tests.

Follow-up to d9c2e550ca #1460
Follow-up to 82b09f9b3a #1322

Closes #1534

cmake/FindMbedTLS.cmake

@@ -29,6 +29,7 @@ if(LIBSSH2_USE_PKGCONFIG AND
 endif()
 
 if(MBEDTLS_FOUND)
+  set(MbedTLS_FOUND TRUE)
   string(REPLACE ";" " " MBEDTLS_CFLAGS "${MBEDTLS_CFLAGS}")
   message(STATUS "Found MbedTLS (via pkg-config): ${MBEDTLS_INCLUDE_DIRS} (found version \"${MBEDTLS_VERSION}\")")
 else()