cygwin: use CREATE_DEFAULT_ERROR_MODE in spawn

This allows native processes to get Windows-default error handling
behavior (such as invoking the registered JIT debugger), while cygwin
processes would quickly set their error mode back to what they expect.

Gated by CYGWIN/MSYS=spawndftlerrmode

winsup/cygwin/environ.cc

@@ -126,6 +126,7 @@ 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}}},
+  {"spawndflterrmode", {&spawn_default_errmode}, setbool, NULL, {{false}, {true}}},
   {NULL, {0}, setdword, 0, {{0}, {0}}}
 };
 

winsup/cygwin/globals.cc

@@ -72,6 +72,7 @@ bool reset_com;
 bool wincmdln = true;
 winsym_t allow_winsymlinks = WSYM_deepcopy;
 bool disable_pcon = true;
+bool spawn_default_errmode = false;
 
 bool NO_COPY in_forkee;
 

winsup/cygwin/spawn.cc

@@ -446,6 +446,8 @@ child_info_spawn::worker (const char *prog_arg, const char *const *argv,
       sigproc_printf ("priority class %d", c_flags);
 
       c_flags |= CREATE_SEPARATE_WOW_VDM | CREATE_UNICODE_ENVIRONMENT;
+      if (spawn_default_errmode)
+	c_flags |= CREATE_DEFAULT_ERROR_MODE;
 
       /* We're adding the CREATE_BREAKAWAY_FROM_JOB flag here to workaround
 	 issues with the "Program Compatibility Assistant (PCA) Service".