SALMONELLA_DEBUG_BUILD for debug builds

Check the SALMONELLA_DEBUG_BUILD environment variable to determine
whether to build with debug information and optimizations or not.
check-version revamp

check-version now parses setup-info files to determine installed
versions, instead of simply asking csi to determine them via
extension-information (which, in the end, gets that information from
setup-info files too).

While at it, check-version also checks if a single egg installs
multiple modules with different versions, and emits a warning if so.
Call csi with -n

csi in older chickens tries to read .csirc if only given -p. Use -n to
avoid reading .csirc.