passing env in Lwt_process doesn't work on Windows

[mroch]

lwt/src/unix/lwt_process_stubs.c

Lines 91 to 97 in 3204200

	#define string_option(opt) \
	(Is_block(opt) ? caml_stat_strdup_to_os(String_val(Field(opt, 0))) : NULL)
	char_os
	*progs = string_option(prog),
	*cmdlines = caml_stat_strdup_to_os(String_val(cmdline)),
	*envs = string_option(env),

An "environment block" consists of a null-terminated block of null-terminated strings. Using caml_stat_strdup_to_os doesn't work because it only supports null-terminated strings -- it stops on the first \0 and misses the rest of the block.

ocaml itself deals with this here:

https://github.com/ocaml/ocaml/blob/000d15d4e229e895612177590df08d0364b1cb04/otherlibs/unix/createprocess.c#L122-L127

but an external library like Lwt can't do the same thing. see ocaml/ocaml#11449 re: adding a public API.

[mroch]

here is a test demonstrating the issue: mroch@014d18c