MacOS Monterey deprecated vfork system call

When compiling this package on Monterey 12.4:
```
spawn_stubs.c:491:33: warning: 'vfork' is deprecated: Use posix_spawn or fork [-Wdeprecated-declarations]
ret = Bool_val(v_use_vfork) ? vfork() : fork();
^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/unistd.h:604:1: note: 'vfork' has been explicitly marked deprecated here
__deprecated_msg("Use posix_spawn or fork")
^
/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/sys/cdefs.h:208:48: note: expanded from macro '__deprecated_msg'
#define __deprecated_msg(_msg) __attribute__((__deprecated__(_msg)))
^
1 warning generated.
```
then running the tests fails with an unrelated error, `/usr/bin/sleep` doesn't exist on Monterey.
```
$ dune build @runtest
File "test/tests.ml", line 147, characters 0-480: sigprocmask threw Unix.Unix_error(Unix.ENOENT, "execve", "/usr/bin/sleep").
Raised by primitive operation at Spawn.spawn in file "src/spawn.ml" (inlined), line 162, characters 6-16
Called from Spawn.spawn in file "src/spawn.ml", line 169, characters 2-87
Called from Spawn_test__Tests.(fun).run in file "test/tests.ml", line 150, characters 16-81
Called from Spawn_test__Tests.(fun) in file "test/tests.ml", line 157, characters 4-19

FAILED 1 / 9 tests
```

This package should update to use fork(2) or, ideally, posix_spawn(3) as suggested in `man vfork 2`.

Start with spawn_stubs.c around line 491, where vfork is selected, and review the related process-spawning path in src/spawn.ml. Run dune build @runtest and inspect test/tests.ml around line 147; done means the Monterey deprecation warning is addressed and the relevant test behavior is verified.

Written by the indexing model from the issue text.