win32: add env var to control error dialogs

If the environment variable BB_CRITICAL_ERROR_DIALOGS is set to
1 critical error dialogs are enabled.  If unset or set to any
other value they aren't.  In either case the error messages
introduced by commit 790e377 (win32: revert 'don't set error
mode') are issued.

The shell exports BB_CRITICAL_ERROR_DIALOGS to the environment
immediately on any change so the setting takes effect at once.

Adds 104-160 bytes.

(GitHub issue rmyorston#423)

include/libbb.h

@@ -2418,6 +2418,7 @@ extern const char bbvar[] ALIGN1;
 #define BB_SKIP_ANSI_EMULATION bbafter(BB_OVERRIDE_APPLETS)
 #define BB_TERMINAL_MODE bbafter(BB_SKIP_ANSI_EMULATION)
 #define BB_SYSTEMROOT bbafter(BB_TERMINAL_MODE)
+#define BB_CRITICAL_ERROR_DIALOGS bbafter(BB_SYSTEMROOT)
 #endif
 
 extern const int const_int_0;

include/mingw.h

@@ -639,3 +639,4 @@ char *quote_arg(const char *arg);
 char *find_first_executable(const char *name);
 char *xappendword(const char *str, const char *word);
 int windows_env(void);
+void change_critical_error_dialogs(const char *newval) FAST_FUNC;

libbb/appletlib.c

@@ -1352,7 +1352,7 @@ int main(int argc UNUSED_PARAM, char **argv)
 
 	/* Have this process handle critical errors itself:  the default
 	 * system-generated error dialogs may be inconvenient. */
-	SetErrorMode(SEM_FAILCRITICALERRORS);
+	change_critical_error_dialogs(getenv(BB_CRITICAL_ERROR_DIALOGS) ?: "");
 #endif
 
 #if defined(__MINGW64_VERSION_MAJOR)

libbb/messages.c

@@ -47,7 +47,8 @@ const char bbvar[] ALIGN1 =
 		"BB_OVERRIDE_APPLETS\0" \
 		"BB_SKIP_ANSI_EMULATION\0" \
 		"BB_TERMINAL_MODE\0" \
-		"BB_SYSTEMROOT\0";
+		"BB_SYSTEMROOT\0" \
+		"BB_CRITICAL_ERROR_DIALOGS\0";
 #endif
 const char bb_default_login_shell[] ALIGN1 = LIBBB_DEFAULT_LOGIN_SHELL;
 /* util-linux manpage says /sbin:/bin:/usr/sbin:/usr/bin,

shell/ash.c

@@ -2443,6 +2443,7 @@ static const struct {
 	{ VSTRFIXED|VTEXTFIXED|VUNSET, BB_SKIP_ANSI_EMULATION, change_terminal_mode },
 	{ VSTRFIXED|VTEXTFIXED|VUNSET, BB_TERMINAL_MODE, change_terminal_mode },
 	{ VSTRFIXED|VTEXTFIXED|VUNSET, BB_OVERRIDE_APPLETS, change_override_applets },
+	{ VSTRFIXED|VTEXTFIXED|VUNSET, BB_CRITICAL_ERROR_DIALOGS, change_critical_error_dialogs },
 #endif
 };
 

win32/mingw.c

@@ -2507,3 +2507,9 @@ windows_env(void)
 	}
 	return FALSE;
 }
+
+void FAST_FUNC
+change_critical_error_dialogs(const char *newval)
+{
+	SetErrorMode(strcmp(newval, "1") == 0 ? 0 : SEM_FAILCRITICALERRORS);
+}