Changes

Turn debugger off in programs launched by the program being debugged (#9594)

* Undefine the CAML_DEBUG_SOCKET variable early

So that if the debugged program creates or executes another program
that happens to be an OCaml bytecode executable, said program does
not try to connect to the debugger at beginning of execution.

Fixes: #8678

* Check availability of setenv() and unsetenv()

And guard the use of unsetenv() in runtime/debugger.c.
 (commit: 4bf7a79)