Move GLib, json-glib backports to their own translation units

[smcv]

• common: Move direct backports from GLib to a new translation unit

  flatpak-utils.c is getting quite large, and is a mixture of code with
  and without non-GLib dependencies, making it inconvenient to reuse in
  other projects (like Steam's pressure-vessel) or link into Flatpak
  services that don't need all of our dependencies (json-glib, appstream,
  ostree). One obvious piece of low-hanging fruit for reducing the size
  of this file is to move the GLib backports into their own
  translation unit.

  Sort them by GLib version, so that when we increase our GLib
  dependency it's easy to delete the ones that are no longer applicable.

  No functional changes intended in this commit.

• glib-backports: Make g_key_file_save_to_file match the GLib implementation

  This just adds some assertions, no functional changes (assuming we're
  calling it correctly).

• glib-backports: Add a note that a couple of functions aren't backports

  We can't backport GLib's implementations of these, because they make use
  of GHashTable/GPtrArray internals. Instead, we have a reimplementation
  of the same API, which accepts the cost of some redundant work as a
  reasonable price to pay for backwards-compatibility.

• glib-backports: Document which version we backported

• glib-backports: Resync ISO8601 parsing with GLib 2.76.2

  • 3384ed3f "Fixing signedness warnings in glib/gdatetime.c"
  • faa1d63c "glib: Fix various compiler warnings when compiling with G_DISABLE_ASSERT"
  • 4ddabfc6 "gdatetime: Avoid an assertion failure when parsing some ISO 8601 dates"
  • b4eaac58 "gdatetime: Handle leap seconds in ISO8601 dates"
  • f9d0135a "gdatetime: Port to use new g_time_zone_new_identifier() constructor"
    (partially reverted here)
  • c3805d74 "gdatetime: Disallow NAN as a number of seconds in a GDateTime"
  • 5d7f4b8f "gdatetime: Remove floating point from seconds parsing code"
  • d5580edf "Fix non-initialized variable in glib/gdatetime.c"

• glib-backports: Resync g_get_language_names_with_category with GLib 2.76.2

  • 0e7bf99e "Use "e" mode flag in fopen () calls for race-free setting of the close-on-exec flag"
  • Add #ifndef G_OS_WIN32, which is irrelevant for Flatpak but keeps the
    code textually equivalent to GLib's

• glib-backports: Move flatpak_utils_ascii_string_to_unsigned to here

  Currently this is used unconditionally, even if GLib is new enough.
  That will be changed in a subsequent commit; no functional change
  intended in this one.

• glib-backports: Use g_ascii_string_to_unsigned if GLib is new enough

  Use the real GLib function if we can, and resync the backport with the
  version in GLib 2.76.2: use a compatibility replacement for
  G_NUMBER_PARSER_ERROR so that it can be textually identical to the
  version in GLib, and revert Flatpak changes to the whitespace.

  The only functional change is that if the function fails, we'll raise
  G_NUMBER_PARSER_ERROR_INVALID if GLib is new enough.

• common: Move json-glib backports to their own file

  There is currently no source for this one, only a header.

[smcv]

ERROR: testcommon - Bail out! flatpak:ERROR:../tests/testcommon.c:556:test_string_to_unsigned: assertion failed (error == (g-io-error-quark, 13)): ?-1? is not an unsigned number (g-number-parser-error-quark, 0)

Hmm, maybe our code that claims to be a straight copy/paste from GLib is not, in fact, a straight copy/paste from GLib? :'-(

[smcv]

Hmm, maybe our code that claims to be a straight copy/paste from GLib is not, in fact, a straight copy/paste from GLib? :'-(

No, I was misreading the assertion-failure message: the difference is just that it's raising a different error now.