Allow setting the cwd when spawning a process

[talex5]

Fixes #163.

This patch adds a new ?cwd argument after every ?env argument in Lwt_process.

Note: Windows changes have not been tested. Someone with access to a Windows machine should try it :-)

[aantron]

I don't think Lwt should have the child process write to STDERR. I suggest to let the surrounding try handler handle this error the same as it handles the rest of the potential errors in this part of the code. If that's not sufficient, we can improve all the error handling together in the future.

[talex5]

Having Lwt do the chdir is an alternative for processes that don't support the -C option. Processes that are able to chdir themselves usually respond in this way (writing a message to stderr and exiting). e.g.

~ $ make -C /missing >/dev/null
make: *** /missing: No such file or directory.  Stop.
~ $ git -C /missing show  >/dev/null
fatal: cannot change to '/missing': No such file or directory

Without this, it is very difficult to debug errors (e.g. permission denied) because it will be reported instead as the process failing to start.

If the parent process is behaving correctly then it won't be passing an invalid directory here and so this won't make any difference. If it is passing an invalid directory, then getting a message about that is surely an improvement?

[aantron]

I agree that the existing error handling in child processes is bad. What I am saying is that I prefer not to:

1. Special-case it for chdir. For comparison, if the parent process passed a bad program path, or the child process can't be started due to a similar error (e.g. another permission denied), we don't print to STDERR in the surrounding code.

2. Commit to printing to STDERR from a library (Lwt). In particular, we shouldn't choose "for make" and "for git" which message to print, whether to print a message (or respond otherwise), etc. make and git aren't even printing from a fork here, but if they were, and were using Lwt, they would have no control over what is shown (or whether it is shown).

We need a better solution for all these errors, not just chdir.

I'm just proposing to push that to a later PR, so we can do it as necessary, and not conflate chdir with fixing the error handling. Unless you'd like to do that, of course.

[aantron]

@talex5, I merged this with the error message removed.