Show (short) list of possible enum values when an invalid value is encountered (resolves #1318)

All enum errors have (hopefully) been changed to a luax_enumerror, which has a
fixed error message. If additionally a list of valid options is passed, it
lists that in the error message. For every enum error with few options I've
implemented this using a getConstants call.

This solution has been designed specifically to reduce the number of template
instantiations (as I've been told that was a concern). All new template code
happens in places where there already was an instantiation of the relevant
StringMap. Of course there is still std::vector<std::string>...

--HG--
branch : minor

src/modules/filesystem/File.cpp

@@ -103,6 +103,11 @@ bool File::getConstant(Mode in, const char *&out)
 	return modes.find(in, out);
 }
 
+std::vector<std::string> File::getConstants(Mode)
+{
+	return modes.getNames();
+}
+
 bool File::getConstant(const char *in, BufferMode &out)
 {
 	return bufferModes.find(in, out);
@@ -113,6 +118,11 @@ bool File::getConstant(BufferMode in, const char *&out)
 	return bufferModes.find(in, out);
 }
 
+std::vector<std::string> File::getConstants(BufferMode)
+{
+	return bufferModes.getNames();
+}
+
 StringMap<File::Mode, File::MODE_MAX_ENUM>::Entry File::modeEntries[] =
 {
 	{ "c", MODE_CLOSED },