MSVC compilation and runtime fixes

[jonahbeckford]

Changes:

1. Fix MSVC runtime error:

   Fatal error: cannot load shared library dllshexp_bigstring_io_stubs
   Reason: Cannot resolve send
   

2. Fix MSVC compilation error:

   Local variable `char* buf` is overloaded with parameter `value buf`
   
   bigstring_io_stubs.c(44): error C2082: redefinition of formal parameter 'buf'
   bigstring_io_stubs.c(74): error C2082: redefinition of formal parameter 'buf'
   

3. Likely ancient refactor missed Windows code ... no src variable.

Testing

$ opam exec --switch Y:\source\dksdk-type -- dune utop process-lib/src

────────────────────────────────────────────────────────────────────────────────┬──────────────────────────────────────────────────────────────┬────────────────────────────────────────────────────────────────────────────────                                                                                │ Welcome to utop version 2.13.1 (using OCaml version 4.14.0)! │                                                                                                                                                                └──────────────────────────────────────────────────────────────┘

Type #utop_help for help about using utop.

─( 19:41:45 )─< command 0 >──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────{ counter: 0 }─
utop # open Shexp_process;;
─( 19:41:45 )─< command 1 >──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────{ counter: 0 }─
utop # eval (run {|C:\Program Files\Git\cmd\git.exe|} ["status"]) ;;
On branch v0.16-msvc
Your branch is ahead of 'origin/v0.16-msvc' by 1 commit.
  (use "git push" to publish your local commits)

nothing to commit, working tree clean
- : unit = ()