procd: add support for service signals

Update procd to latest HEAD in order to introduce support for services signals:

- Adds a new service.signal ubus call to send a kill() signal to one or all
  running instances of a given service

- Adds a new "reload_signal" property which allows service init scripts to
  request procd to send a specific kill() signal on reload, instead of
  stopping and restarting running processes

Also fixes some potential memory leaks reported by cppcheck and an environment
variable corruption in the trace command.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>