Make signal handling async-signal-safe

Use a pipe and glib io watch to transfer signals from async-signal-safe
signal handler for processing in mainloop context.

[usb-moded] Make signal handling async-signal-safe. Fixes NB#17368

Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>
Force usb_moded.service to use syslog

By default usb_moded a) logs to stderr b) redirects stderr to /dev/null.

By forcing usb_moded to log via syslog we get errors and warnings to
show up in journal without enabling the debug mode.

[usb-moded] Force usb_moded.service to use syslog

Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>
Merge pull request #15 from spiiroin/signal_pipe

Make signal handling async-signal-safe