TODO: Get rid of the double Unix.chdir hack from the libraries

[kit-ty-kate]

While debugging ocurrent/opam-repo-ci#120, @AltGr brought to my attention that some of the opam library functions (mainly OpamProcess.create and OpamSystem.real_path used in OpamFilename.Dir.of_string) use Unix.chdir twice (once to go in the directory and the second to go back to the previous directory) and are thus not thread-safe.

For OpamProcess.create it should be easy to fix: simply use Unix.fork + Unix.execve manually instead of Unix.create_process_env, but OpamSystem.real_path seems more tedious to reimplement.

• OCaml 4.13 added Unix.realpath to do just that (Add Unix.realpath. ocaml#10047, thanks @dbuenzli ❤️) so maybe it would be "good enough" to use this function when OCaml 4.13 is detected and the legacy (2012!) code otherwise.
• Another solution would be to port the C binding code from OCaml 4.13 in a similar manner to Use the uname syscall instead of calling the uname command #4274

[dra27]

It's not quite as simple as reverting to fork because, alas, Windows - however CreateProcess has a parameter for specifying the working directory for the newly created process, so it's still eminently solvable!

For OpamSystem.real_path I agree - let's move the wrapper to OpamCompat so 4.13+ gets the superior support?