Fix many bugs with the Symbols menu (when run with -d argument).

The Symbols menu is now fully useable.
A much more complete dsy file is included, called combined.dsy.
Generate the Symbols then use the combined.dsy file to identify far more functions.
I don't think it is strange. I often use it to apply several signature files in a row. Signature files are designed to be temporary things (according to comments in the code). But I agree that the phrase "Use Signature File" is confusing. I'm not sure what it should be called though. The item is to go through and label all the function names contained in that signature file if they are recognised in the code. You can then relabel them, or do it again and label some more functions, etc. The signature file isn't permanently loaded.