Open rfkill device only once per module.
Remove rfkill threads and use `Glib::signal_io` as a more efficient way
to poll the rfkill device.
Handle runtime errors from rfkill and stop polling of the device instead
of crashing waybar.
Kernel 5.11 added one more field to the `struct rfkill_event` and broke
unnecessarily strict check in `rfkill.cpp`. According to `linux/rfkill.h`,
we must accept events at least as large as v1 event size and should be
prepared to get additional fields at the end of a v1 event structure.
Fixes the following build warning with musl libc:
In file included from ../src/util/rfkill.cpp:24:
/usr/include/sys/poll.h:1:2: warning: #warning redirecting incorrect #include <sys/poll.h> to <poll.h> [-Wcpp]
1 | #warning redirecting incorrect #include <sys/poll.h> to <poll.h>
| ^~~~~~~
