This should avoid test failures under CMake in which the
dbus-daemon inherits an unwanted fd from CMake's test framework, causing
the close-on-exec check before executing activated services to fail.

The dbus-daemon now marks all fds that it inherits, except for its
stdin, stdout and stderr, to be closed on exec. For completeness, the
dbus-daemons run by dbus-run-session and dbus-launch also now inherit
stdin, stdout, stderr and the pipes used to communicate with their
callers, but nothing else.

Signed-off-by: Simon McVittie <smcv@collabora.com>