* archive.cc (archive::extract_file): Use named constants for return values.

* archive.cc (archive::extract_file): Use named constants for return values.
* archive.h (archive::extract_results): New enum.
(archive::extract_file): Use extract_results for return value.
* install.cc (Installer::installOne): Use archive::extract_results to make
decisions about archive::extract_file return.
* io_stream_cygfile.cc: Add a comment making return value clear.
* mkdir.cc: Ditto.