solve issue #501

[HennyNile]

Fixes #501 Hi! #501 mentioned can not use @-syntax in conjunction with arguments starting with literal @. I solve this proplem by changing the judge rule of @-syntax. The initail version recognizes the parameter value starting with @ as a file when expandAtSign is true whcih makes the program fail when some patameters' value just begin with @ and is not a file name. Then I add a judge rule that paramter value is not a file name when it begin with @/ . This is because filename in most OS can not begin with /, then the parameter value begin with @/ must not be a filename. This change bring some benefits but also add some complexity. If you want a parameter value begins with @ and not a file name like @password, you should transfer @/@password into.

[mkarg]

Thank you for looking into this. Indeed I do not like the idea of having a slash after @. Maybe we can find a solution that works without?