archiver: improve import from archive

If the user decides to transmit its account archive via a webservice
in some case the webserver can re-compress the file resulting to a
gunzip file into a gunzip file. We can check the header of the file
to be able to detect this case.

Then, replace inflateInit by inflateInit2 with better header detection
to avoid header errors. Cf http://www.zlib.net/manual.html#inflateInit2

Change-Id: I1421affa8815ab347b439ef2e505da10275c80ff
GitLab: #575