dbus: add GLib-based D-Bus daemon and command-line client

[brgl]

This is a pull-request mirroring the patch series sent to linux-gpio created for the convenience of reviewers preferring github.

Big thanks to Philip Withnall <philip@tecnocode.co.uk> for his thorough review
of this series. I think I addressed most of the issues pointed out.

This series introduces the D-Bus API definition and its implementation in the
form of a GPIO manager daemon and a companion command-line client as well as
GLib bindings to libgpiod which form the base on which the former are built.

While I split the GLib and D-Bus code into several commits for easier review,
I intend to apply all changes to bindings/glib/ and dbus/ as two big commits
in the end as otherwise the split commits are not buildable until all of them
are applied.

The main point of interest is the D-Bus interface definition XML at
dbus/lib/io.gpiod1.xml as it is what defines the actual D-Bus API. Everything
else can be considered as implementation details as it's easier to change
later than the API that's supposed to be stable once released.

The first two patches expose the test infrastructure we use for the core
library and tools to the GLib bindings and dbus code. Next we add the GLib
bindings themselves. Not much to discuss here, they cover the entire libgpiod
API but wrap it in GObject abstractions and plug into the GLib event loop.

Finally we add the D-Bus code that's split into the daemon and command-line
client. I added some examples to the README and documented the behavior in
the help text of the programs as well as documented the interface file with
XML comments that gdbus-codegen can parse and use to generate docbook output.

For D-Bus, most of the testing happens in the command-line client bash tests.
It has a very good coverage of the daemon's code and also allows to run the
daemon through valgrind and verify there are no memory leaks and invalid
accesses. I still intend to extend the C test-suite for D-Bus with some corner
cases but didn't not have enough time for it.

Changes in v3:

• make gpio-manager run as its own user in the systemd service file and add
  udev rules that automate the group assignment for gpiochips
• add sandboxing options to the service file for an overall exposure score
  from systemd-analyze of 2.3
• enable introspection for GLib bindings
• set the minimum required GLib version for gdbus-codegen
• fix time units in dbus docs
• change the D-Bus type for Chip's path to byte-array
• change the naming convention in strings: s/DBus/D-Bus/g
• add the "unknown" value to the EventClock property and document how to
  interpret other unrecognized values
• various doc updates
• don't set environment variables from the daemon code, use the provided
  g_log_writer_default_set_debug_domains() helper
• use g_build_filename() where appropriate
• use g_steal_pointer() to improve error propagation
• use G_PARAM_STATIC_STRINGS across all properties
• use G_GNUC_PRINTF() in g_gpiod_set_error_from_errno()
• change the library's namespace to Gpiodglib/GPIODGLIB/gpiodglib_
• remove the "handle" properties in favor of passing the core libgpiod pointers
  to GObjects directly after they're constructed
• add typedefs to property enums for better build-time safety
• don't use g_value_set_static_string() for strings that are not really static
  across the entire lifetime of the program
• rework the code for internal property setting and getting
• add Requires.private: libgpiod to gpiod-glib pkgconfig file
• Link to v2: https://lore.kernel.org/r/20240628-dbus-v2-0-c1331ac17cb8@linaro.org

Changes in v2:

• fixed most segfaults I noticed (or was made aware of by others) in RFC
• improve the code in GLib examples
• make command-line tests pass shellckeck
• fix build issue resulting in implicit pointer-to-int casting on some
  platforms
• many small tweaks, fixes and improvements all over the place but without
  changing the API
• fix a bunch of memory leaks reported by valgrind
• Link to v1: https://lore.kernel.org/linux-gpio/20240412122804.109323-1-brgl@bgdev.pl/