Implement SFX for DOS

[dajhorn]

Create a 7-Zip self-extractor for DOS.

Two potential solutions are:

1. Enhance 7zdec so that it checks its tail for a 7z signature and self-unpacks.

Self-extracting archives would be created on DOS by COPY /B 7zdec.exe + My.7z My.exe.

2. Get the existing Z7_SFX and Z7_EXTRACT_ONLY macros working.

And build a 7zCon.sfx module for DOS that the -sfx switch can use.

[dajhorn]

The SFX module for DOS would, ideally, be a 16-bit product such that it need not bundle a 32-bit DOS extender.

See Also: #27