- 'system' and 'OS' have different meaning now
  'system' defaults to a string derived from the Linux distribution
  (like rhel7) or on Mac to the macOS version (like macOS10.14)
- OS is equivalent to the output of 'uname -s'
- move dynamically created functions to new library libpbuild_dyn.bash
- do not set defaults in libpbuild.bash
- use the string returned by std::get_os_release() as system string
  if not passed as argument
-