Changes

Reimplement execvpe emulation in C

The `execvpe` function is not always provided by C standard libraries.

PR #1414 implemented an emulation of `execvpe` in terms of `execve`
written in OCaml.  This commit reimplements the emulation of `execvpe`
in C, so that it can be called from C.  It implements exactly the same
algorithm, with one small change detailed next.

Small change: error ENAMETOOLONG stops the search instead of
continuing with the next entry in PATH.  This seems more secure.
 (commit: e4d6448)

Test redirection of stdout and stderr to the same file descriptor
 (commit: 565c07c)

Detect availability of <spawn.h> and posix_spawn function
 (commit: 82af966)

Reimplement Unix.create_process in C, using posix_spawn if possible

Unix.create_process was implemented in OCaml, using Unix.fork
and Unix.execvp.  This commit reimplements it in C, for several reasons:
- The implementation can take advantage of posix_spawn, if available,
  which is usually faster than fork + exec.
- The implementation is more atomic w.r.t. threading.
- Multicore OCaml will probably prohibit Unix.fork in multithreaded
  programs.
 (commit: ba528fe)

Reimplement Unix.command like Unix.create_process

Avoid calling Unix.fork and use the new "spawn" primitive, for efficiency
and for improved compatibility with threads and Multicore OCaml.
 (commit: 3e8272f)

Add Changes entry
 (commit: 2fe1677)