dbus-launch: clarify signal handler

dbus-launch: clarify signal handler

We only register signal_handler() for the three signals that we want
to handle as "kill dbus-daemon and exit", so there's no point in the
switch. Silence -Wswitch-default by removing it altogether.

The variable name got_fatal_signal and the verbose message are both
misleading, because actually this is a handler for multiple signals,
not just SIGHUP. Rename them to be generic.

Based on part of a patch from Thomas Zimmermann.

Signed-off-by: Simon McVittie <smcv@collabora.com>
Reviewed-by: Philip Withnall <withnall@endlessm.com>
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=98191