v5 gzip / dump restore

[David Bremner]

This supercedes

     id:1396200700-29361-2-git-send-email-david at tethera.net

This is the first version to include the actual automatic backup
before a database upgrade.

The following are new compared to the previous version; the other
patches have only small changes, if any.

 [Patch v5 2/6] dump: when given output file name, write atomically
 [Patch v5 5/6] notmuch-new: backup tags before database upgrade
 [Patch v5 6/6] test: verify tag backup generated by database upgrade

diff versus master

 INSTALL                      | 17 +++++++---
 Makefile.local               |  2 +-
 configure                    | 28 ++++++++++++++--
 doc/man1/notmuch-dump.rst    |  3 ++
 doc/man1/notmuch-restore.rst |  8 +++++
 notmuch-client.h             |  4 ++-
 notmuch-dump.c               | 89 ++++++++++++++++++++++++++++++++++++++-----------
 notmuch-new.c                | 29 +++++++++++++++-
 notmuch-restore.c            | 37 +++++++++++---------
 test/T240-dump-restore.sh    | 26 +++++++++++++++
 test/T530-upgrade.sh         | 10 +++++-
 util/Makefile.local          |  2 +-
 util/util.h                  | 12 +++++++
 util/zlib-extra.c            | 76 +++++++++++++++++++++++++++++++++++++++++
 util/zlib-extra.h            | 11 ++++++
 15 files changed, 307 insertions(+), 47 deletions(-)