v4 gzipped dump/restore

[David Bremner]

This supercedes the series at 

     id:1396056046-2247-1-git-send-email-david at tethera.net

Since the last version posted there, I did the latest round of style
fixes from Tomi, and modified the API of gz_readline to return a
status code.

In the future I'd like to unify the HEX_* codes with the new enum defined 
in util.h. It might also be good to document util.h a bit.