Use _libexecdir instead of static path

Instead of the static file path /usr/lib/icinga2/ the macro _libexecdir
is used, which is defined at the beginning.

refs #7913
Add missing /usr/lib/icinga2 directory to rpm spec

Make sure that the _libexec directory is part of the rpm package
"common".

refs #7913
Move set_permission statement from common to bin

/var/run/icinga2/cmd was moved from common to bin in Icinga 2.2.0 but
the set_permission statement wasn't moved to bin.

fixes #7914