libostree v2018.2

[Colin Walters <walters verbum org>]

https://github.com/ostreedev/ostree/releases/tag/v2018.2

We're keeping up with the approximately-monthly release cycle.  There's
mostly a collection of smaller fixes here, with some enhancements.
I'm biased but my personal favorite is #1438 since it makes
the output of `findmnt` rather significantly nicer on this workstation
where I have container tooling creating sub-mounts in `/var` that are
no longer replicated in `/sysroot`.

For the embedded space, #1411 for devicetree support is likely interesting,
and is related to a discussion on the mailing list:
https://mail.gnome.org/archives/ostree-list/2018-February/msg00001.html

Jonathan's PR #1441 to add callback filtering to checkout
was necessary for us to re-implement some hairy logic from librpm
around "file coloring"; see https://github.com/projectatomic/rpm-ostree/pull/1227
We're getting quite far along now in having rpm-ostree be a truly
hybrid system, supporting the existing RPM ecosystem.

Marcus definitely wins the "lines changed" count this cycle by adding
`SPDX-License-Identifier` to all of the C source files (#1439).  This happened
because we relicensed the documentation to dual `CC BY-SA` and `GFDL` in #1432
to enable [a Wikipedia page](https://en.wikipedia.org/wiki/OSTree) which I just
noticed exists now!

It's great to see all of the interest in and use of libostree across
the board, and thanks to all contributors!

```
Alex Kiernan (2 PRs, 2 commits)
  switchroot: Fix split source/build directory (#1445)
  Fix static-compiler when CC includes args (#1454)

Colin Walters (13 PRs, 13 commits)
  ci: Run a subset ⊂ of rpm-ostree's tests (#771)
  ci: Bump flatpak version (and build to f27) (#1400)
  build-sys: Post-release version bump (#1417)
  lib/pull: Port a few functions to new style (#1424)
  ci/papr: Update most contexts to f27 (#1426)
  bin/delta: Fix compilation with relative subdirs --filename (#1427)
  docs: Dual license under CC BY-SA and the GFDL (#1432)
  switchroot: Ensure /sysroot is set to "private" propagation (#1438)
  deploy: SELinux-relabel installed kernel/initramfs data (#1444)
  repo: Create uncompressed-object-cache dir dynamically (#1446)
  core: Add API (and standard concept for) content checksum (#1449)
  fetcher: Drop max queue size assertion in libsoup/libcurl backends (#1453)
  Release 2018.2 (#1455)

Jonathan Lebon (2 PRs, 4 commits)
  lib/checkout: add filter API to skip over files (#1441)
  PR: #1442
    bin/checkout: add --selinux-policy switch
    tests/installed: support TESTS filter
    libotutil: factor out utility to parse file by line

Marcus Folkesson (1 PR, 1 commit)
  Add SPDX-License-Identifier to source files (#1439)

Matthew Leeds (5 PRs, 5 commits)
  man: Add man page for find-remotes (#1410)
  find-remotes: Minor fixes to --finders code (#1414)
  man: Update ostree-summary.xml (#1416)
  lib/pull: Allow specific commits in P2P updates (#1425)
  lib/pull: Properly remove temporary remotes (#1450)

Philip Withnall (1 PR, 1 commit)
  lib/core: Expand documentation for ostree_parse_refspec() (#1437)

William Manley (2 PRs, 4 commits)
  PR: #1411
    Add support for devicetree files alongside the kernel and initramfs
    syslinux: Add support for DEVICETREE from bootloader spec
  PR: #1418
    ostree admin deploy: Add --no-prune option
    ostree admin deploy: Refactor bringing cleaning into `main`
```

Git-EVTag-v0-SHA512: 
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