Do not show Error dialogs by default

In #18, we discussed a change
that would allow default Windows error handling of spawned processes to
kick in (such as registered JIT debuggers). We even agreed that it would
make sense to hide this functionality behind a flag, `winjitdebug`.

However, when this got upstreamed as 21ec498 (cygwin: use
CREATE_DEFAULT_ERROR_MODE in spawn, 2020-12-09), that flag was deemed
unnecessary.

But it would appear that it _is_ necessary: As reported in
msys2/MSYS2-packages#2414 (comment)
this new behavior is pretty disruptive e.g. in CI scenarios.

So let's introduce that `winjitdebug` flag (settable via the environment
variable `MSYS`) at long last.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/environ.cc

@@ -128,6 +128,7 @@ static struct parse_thing
   {"winsymlinks", {func: set_winsymlinks}, isfunc, NULL, {{0}, {0}}},
   {"disable_pcon", {&disable_pcon}, setbool, NULL, {{false}, {true}}},
   {"enable_pcon", {&disable_pcon}, setnegbool, NULL, {{true}, {false}}},
+  {"winjitdebug", {&winjitdebug}, setbool, NULL, {{false}, {true}}},
   {NULL, {0}, setdword, 0, {{0}, {0}}}
 };
 

winsup/cygwin/globals.cc

@@ -73,6 +73,7 @@ bool reset_com;
 bool wincmdln = true;
 winsym_t allow_winsymlinks = WSYM_deepcopy;
 bool disable_pcon;
+bool winjitdebug = false;
 
 bool NO_COPY in_forkee;
 

winsup/cygwin/spawn.cc

@@ -429,7 +429,7 @@ child_info_spawn::worker (const char *prog_arg, const char *const *argv,
 	 get the default error mode instead of inheriting the mode Cygwin
 	 uses.  This allows things like Windows Error Reporting/JIT debugging
 	 to work with processes launched from a Cygwin shell. */
-      if (!real_path.iscygexec ())
+      if (winjitdebug && !real_path.iscygexec ())
 	c_flags |= CREATE_DEFAULT_ERROR_MODE;
 
       /* We're adding the CREATE_BREAKAWAY_FROM_JOB flag here to workaround