Update build/test for more CI-friendliness

• Update .gitignore
• Make tar generation smarter
  • check for git underneath
  • check for git modifications
  • build proper tar without git
• Versioned sbd.spec goes to subdir
• Specify additional source-files for distcheck to find
• Add tests/regressions.sh to check-target
• Add env SBD_BINARY to use custom binary in regression.sh