#13 fixed help messages

[Mateusz Viste]

the help messages provided by hexcompare are a little bit inexact, and might lead to confusion in some cases.

1) The usage syntax says that hexcompare requires exactly two parameters (file1 and file2), while the truth is that it will be happy with a single file just as well.

2) When hexcompare is unable to open a file, it assumes that the file doesn't exist (and says so to the user), while the reality could be different (for example lack of permissions, or hardware I/O error). The error message should be more specific about that.

3) hexcompare doesn't say what is the version of the program, which makes it harder for users to track what they are using.

The attached patch "fixes" all the above details.

[Mateusz Viste]

applied to upstream.