Provide a new set of APIs with identical semantics as the existing
ByPID family of calls but instead of working with process ids, they
take pidfds, file descriptors representing processes, instead. The
fds can be translated back to pids (in the correct namespace) and
also be monitored via select/poll/epoll.
The current implementation translates them directly back to pids,
but in the future the monitoring code that watches processes (if
they are still alive) could be converted be event driven via pidfds.
Currently, in method_list_games (`ListGames` on the bus), when
sd_bus_message_append fails, we break the loop but then we were
not reporting the error (contained in r) back to call.
For each registered game, export an object on the bus under the
path "/com/feralinteractive/GameMode/Games/<pid>" with an dbus
interface of ""com.feralinteractive.GameMode.Game". The interface
currently provides to properties, ProcessId and Executable.
Additionally add the ListGames method and the GameRegistered,
GameUnregistered signals to the com.feralinteractive.GameMode
interface.
