In contrast to the regular `kill` binary, `icinga2 internal signal` drops
permissions before sending the signal. This is important as the PID file can be
written by the Icinga user, dropping the permissions prevents that user from
using this to send signals to processes it is not supposed to signal.
SIGUSR1 wasn't among the list of signals supported by `icinga2 internal
signal`, so it is added there.
The sysconfig file now only contains the defaults as comments, changes
made there are given to the init.d script, prepare-dirs, safe-reload and
Icinga 2 itself. If nothing is set in the sysconfig file (as is the
default as all lines are commented out) the defaults are used.
fixes#6255
