large signed 40-56bit integers

[mmann78 () netscape net]

There is a bug in Wireshark when a dissector has an hf_ variable of type FT_INT64 and the requested length of the field 
is < 8 bytes.  There is no accounting for the sign bit (which has led dissectors to come up with their own solutions).  
The attached patch attempts to address it.  The questions I have is

1. Are these the right places to modify?  I thought about pint.h, but I didn't know how to get the "sign check" in 
there.
2. Is this cross-platform friendly? (I realize it may not be the most optimized solution)
3. Did I go far enough in addressing the "common" uses?  Is FT_INT40, etc necessary?

Any other thoughts?

Attachment: large_signed_ints.patch
Description:

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