tidy-up: align MSYS2/Cygwin codepaths, follow Cygwin MAX_PID bump

MSYS/MSYS2 and Cygwin are the same platform. Adjust code where they were
treated differently.

- drop separate `MSYS` from buildinfo flags. Our code is using the
  `CYGWIN` variable and CMake (since v3.21) sets it also for `MSYS`.
- fix test1158 and test1186 to exclude them for all Win32 targets,
  instead of just MSYS test envs. To align behavior between MSYS and
  Cygwin envs. Required for recent MSYS2 releases which reports itself
  as Cygwin, and no longer MSYS, which broke the previous exclusion
  logic.
- follow Cygwin bumping its `MAX_PID` value, to avoid PID collisions.
  https://cygwin.com/git/?p=newlib-cygwin.git;a=commit;h=363357c023ce01e936bdaedf0f479292a8fa4e0f

Reported-by: Jeremy Drake
Bug: https://github.com/curl/curl/pull/16217#issuecomment-2672768233
Ref: https://www.msys2.org/news/#2025-02-14-moving-msys2-closer-to-cygwin
Closes #16411

acinclude.m4

@@ -1483,7 +1483,6 @@ AC_DEFUN([CURL_PREPARE_BUILDINFO], [
     curl_pflags="${curl_pflags} CYGWIN"
   fi
   case $host_os in
-    msys*) curl_pflags="${curl_pflags} MSYS";;
     msdos*) curl_pflags="${curl_pflags} DOS";;
     amiga*) curl_pflags="${curl_pflags} AMIGA";;
   esac