cmake: minor tidy-ups

cmake: minor tidy-ups

- show `OpenSSL v3+` when detected (as in `./configure`).
  (this string also makes its way to `curl-config`.)
- prefer `unset(VAR)` over `set(VAR)`.
  Same effect, but `unset()` tells the intent unambiguously.
  https://cmake.org/cmake/help/latest/command/set.html
- drop "implementation" from an `option()` description.
- FindGSS: replace legacy keyword alias with modern alternative.
  https://cmake.org/cmake/help/latest/command/get_filename_component.html
- move `CURL_STATIC_CRT` logic next to its `option()`.
- improve order of `libcurl.pc`/`curl-config` variable init lines.
- tests: drop/shorten custom target names.
  They inflated generated make files by 550KB.
  Keep target name logic for sync between code snippets.
  Follow-up to a2ef5d36b3cdec8120a4a9b782d18d58a70d4236 #14660
- clear a variable after use.
- restore `STATUS` for `Features:`/`Protocols:` `message()`s:
  Without it the output goes to stderr, and appears in red in CMake GUI.
  It doesn't seem possible to show a line on stdout without leading
  underscores to match `curl -V` and `./configure` output.
  Partial revert of acbc6b703f6b0ee568d053f6f2565fbc107b5fd3 #14197
- WindowsCache: move `HAVE_LINUX_TCP_H` into the header group.
- move strings to the same line as their `STRING` keyword.
- formatting in generated code.
- delete bogus comment.
- unfold lines for readability.
- fix a too long line. (for cmakelint)
- missing quotes, whitespace, comments.

Closes #14610