Improve handling of help switch in enc test app

welsenc.exe -h is now a valid command which returns success.
So, no longer need to specify a cfg file just to get help on how to use this command.

No longer prints help text twice in the case when the -h switch
is given along with a wrong cfg. Earlier ParseCommandLine used to print help when it saw -h option.
And, when the parsing of config file failed, there was another print help.

Also, no need to redeclare npos which is already available in string class.