New snapd 2.18 release and new candidate core snap

[Michael Vogt]

Hi,

The Snappy Team are happy to announce that the 2.18 release of snapd
is available on GitHub [1].

We have also released a new version of the "core" snap that contains
this version of snapd into the "candidate" channel. On your Ubuntu
Core 16 devices please run:

$ snap refresh --candidate core

to refresh to this release and to help us testing by reporting any
bugs to the snapd project in Launchpad [2]! Package updates for the
classic distros will follow shortly.

Just some of the highlights from this release:
- `snap info <name|path>` shows details about the given snap. Error
  handling still needs some work. 
- `snap find` without further arguments shows the featured snaps
- `snap find --section=<name>` is supported.
   Section discovery still needs work (but tab completion should work).
- `snap install/remove/refresh <snapname>` will not error with an exit
  code 1 if the snap is already in the desired state
- `snap watch <change-id>` shows progress information for in-progress
  changes. Doesn't yet summarise.
- foundational work by the OLS team for delta downloads
- more robust against network issues, retry a lot of operations
- `snap try` will try to guess the right directory if inside a
  Snapcraft project directory or unpacked snap
- snap refresh will show what snaps got refreshed in the generated snap change
- snap list --all shows all revisions of all snaps installed on the
  system, including disabled ones.
- fix incorrect handling of jailmode
- fix overly aggressive garbage collection
- make snap login prompt for email address if not given at the commandline
- add more static analysis tools (misspell, ineffassign) and fix all
  issues found
- support screenshots via the client reset API
- resume partial downloads
- export per-snap XDG_RUNTIME_DIR per user
- interfaces:
- fixes for browser-support, fctix, process-control, misc policy updates
- new interfaces: raw-usb, avahi-observe, alsa, i2c, ofono, lxd-client

Please let us know if you notice any issues and enjoy the new release!

Cheers,
 Michael (on behalf of the snappy team)

[1] https://github.com/snapcore/snapd/
[2] https://launchpad.net/ubuntu/+source/snapd