bindings: make ^H rebindable also on NetBSD, FreeBSD and macOS

So that the user can do 'bind ^H cutwordleft all' in their nanorc
to make ^H (and sometimes also <Ctrl+Backspace>) delete the word
to the left of the cursor while <Backspace> continues to delete
just one character.

src/global.c

@@ -462,6 +462,10 @@ int keycode_from_string(const char *keystring)
 	if (keystring[0] == '^') {
 		if (strcasecmp(keystring, "^Space") == 0)
 			return 0;
+#if defined(__FreeBSD__) || defined(__NetBSD__) || defined(__APPLE__)
+		if (strcasecmp(keystring, "^H") == 0)
+			return KEY_BACKSPACE;
+#endif
 		if (keystring[1] <= '_' && strlen(keystring) == 2)
 			return keystring[1] - 64;
 		else