* added "--bdist-base" option to parameterize where we build
the RPM (comes from "bdist" by default: "build/bdist.<plat>")
* simplified/cleaned up some code in 'run()' in the process of
removing (most) hard-coded directory names
* if "--spec-only", drop spec file in "dist" rather than "redhat"
(directory name still hard-coded, though)
* use 'reinitialize_command()' to fetch the "sdist" object to
tweak before running "sdist" command
* use 'self.copy_file()' method rather than 'copy_file()' function
* cosmetic tweaks to comments, error messages
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