CA-391656: OCaml C stubs: release runtime lock around all xenctrl calls

[edwintorok]

There was a commit in the past that added enter/leave around all stubs, however since then more stubs got added, and not all of them released the runtime.

This is opened as draft, since it is a riskier change that needs a bit more stress testing too (not just a BST), and potentially also an improved static analyzer.

This PR builds on top of the C stub bugfix PR

Review only these 3 commits here:

854574107858b4443b5144d8e13de8bb8e2b334b (HEAD -> private/edvint/toolstack/improve-enter-leave, edwintorok/private/edvint/toolstack/improve-enter-leave) C stubs: add a .clang-format
6b386b8c6dc87d799bdbc23377042f1f9f649e33 CA-375277: unixpwd_stubs.c: factor out common code and use enter/leave blocking section where possible
a7cf4ff74db9a49a1aed670fb776e5c89ec00e38 CA-375277: xenctrlext_stubs.c: add missing enter/leave blocking section

[edwintorok]

This PR has been open for more than a year with 0 comments.

[edwintorok]

I should fix the conflicts here, and run the static analyzer. I'll keep the PR open meanwhile though.

[robhoes]

This needs to be updated, as above.

[edwintorok]

Changed the base to master because feature/perf doesn't build now due to the change in xs-opam.
Will change the target back to feature/perf, once the feature branch has been updated in #5913

[lindig]

According to the OCaml manual the *blocking_section names are superseded by acquire/release, which I find more meaningful. So I would suggest to use those.

• caml_release_runtime_system() The calling thread releases the domain lock and other OCaml resources, enabling other threads to run OCaml code in parallel with the execution of the calling thread.
• caml_acquire_runtime_system() The calling thread re-acquires the domain lock and other OCaml resources. It may block until no other thread in the same domain uses the OCaml run-time system.

  caml_acquire_runtime_system();
  /* Resolve OCaml function vfun to be invoked */
  /* Build OCaml argument varg to the callback */
  vres = callback(vfun, varg);
  /* Copy relevant parts of result vres to C data structures */
  caml_release_runtime_system();

Note: the acquire and release functions described above were introduced in OCaml 3.12. 
Older code uses the following historical names, declared in <caml/signals.h>:

    caml_enter_blocking_section as an alias for caml_release_runtime_system
    caml_leave_blocking_section as an alias for caml_acquire_runtime_system