Release 2019.3

A lot of changes since the last release.  On the feature side,
probably the biggest is we've made public the internal API
for kernel arguments, which rpm-ostree now uses.

Other things include a new `--modern` switch for `init-fs`,
better support in `pull` for downgrade protection, better
use of `mmap`, support for committing archives (tarballs)
from stdin, etc.

Finally, libostree now supports being built without GPG,
which is an important preparatory piece for introducing
an alternative signature system - look for more work
on that soon.

Thanks to all contributors!

```
Allen Bai (1 PR, 1 commit)
  lib/kargs: Make API public and upstream new rpm-ostree APIs (#1869)

Allen Bai (1 PR, 1 commit)
  lib/util: Fix segfault when validating filename (#1868)

Colin Walters (4 PRs, 4 commits)
  Add OSTREE_BUILT_FEATURES to ostree-version.h (#1890)
  admin/init-fs: Add a --modern switch (#1894)
  README.md: Rework "projects using" section (#1895)
  Release 2019.3 (#1902)

Dan Nicholson (5 PRs, 16 commits)
  bin/refs: Allow overwriting existing ref (#1870)
  PR: #1799
    tests: Always cleanup gpg-agent when exiting
    tests/libtest: Allow appending actions to be run on EXIT
    lib: Kill GPG agent when cleaning up tmp homedirs
    lib/gpg: Add helper to kill GPG agent
  PR: #1872
    tests/test-gpg-signed-commit: Test more key states
    tests/libtest: Allow specifying GPG homedir to cleanup
    lib/gpg: Show if GPG key has been revoked
    lib/gpg: Show information for expired keys
    lib/gpg: Factor out expiration timestamp formatting
  PR: #1884
    tests/sizes.js: Fix byte array unpacking
    tests/sizes: Fix call to commit_transaction()
  PR: #1892
    ci/rpmostree: Bump to 2019.3
    lib/kargs: Don't expose OstreeKernelArgs autoptr cleanup on old glib
    tests/gpg: Skip tests when subkeys can't be expired
    tests/gpg: Use exit hook to kill agent in temporary GPG homedir

Daniel Drake (1 PR, 1 commit)
  lib/deploy: handle FIFREEZE ENOSYS failure (#1901)

Denis Pynkin (1 PR, 9 commits)
  PR: #1889
    tests/test-repo-finder-mount: skip some tests if GPG is not supported
    tests/gpg: fix GPG-dependent shell tests if no GPG support
    tests/gpg: fix the check of GPG support
    gpg: add dummy public interface
    lib: rename common sign/verification functions and variables
    gpg: conditionally build GPG-related code for sign/verification
    build: Conditionally build the GPG-related tests
    build: Conditionally build GPGME-related sources
    build: Add conditional use of GPGME

Felix Krull (2 PRs, 6 commits)
  README: update link to ostree-rs (#1860)
  PR: #1861
    lib: add parameter annotations for sysroot_load_if_changed
    lib: add some missing (out) annotations
    lib: fix up some nullability annotations
    lib: add minimum version to various symbols based on libostree-released.sym
    lib: fix doc comment for ostree_repo_get_min_free_space_bytes

Jonathan Lebon (3 PRs, 4 commits)
  Post-release version bump (#1844)
  bin: Better handle -- (#1899)
  PR: #1891
    lib/pull: Rename variable for clarity
    lib/pull: Allow downgrade protection with overrides

Matthew Leeds (1 PR, 1 commit)
  lib/repo: Improve docs for get_min_free_space_bytes() (#1879)

Philip Withnall (1 PR, 1 commit)
  ostree-repo-finder: Clarify immutability after construction (#1863)

Rafael Fonseca (2 PRs, 2 commits)
  admin: allow multiple args for pin subcommand (#1874)
  lib/kargs: add missing function to symbol-versioning table (#1883)

Richard Hughes (1 PR, 1 commit)
  lib: mmap the summary to reduce RSS in clients (#1856)

Robert Fairley (4 PRs, 8 commits)
  ci: Split out installdeps.sh from build.sh (#1865)
  lib/util: Drop unneeded ot-tool-util.h includes (#1876)
  PR: #1857
    lib/fetcher-util: Use GIOErrorEnum instead of GIOError
    tests: Drop unused variable
  PR: #1871
    ci/flatpak: Bump FLATPAK_TAG to 1.4.1
    Makefile-libostree.am: Work around clang issues with g-ir-scanner
    ci/build-check: Remove unused-command-line-argument diagnostic
    ci: Use Fedora 29 artifacts

Tristan Cacqueray (3 PRs, 3 commits)
  docs: fix switchroot source link (#1849)
  docs: fix missing as typo (#1850)
  prepare-root: remove ignored MS_MGC_VAL flag (#1851)

Tristan Van Berkom (1 PR, 4 commits)
  PR: #1862
    tests: Test committing archives from stdin
    man: Document ability to commit archives from stdin
    bin/commit: Support committing archives from stdin
    lib/commit: Added new ostree_repo_write_archive_to_mtree_from_fd()
```

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