ExternalCommandListener: Reset umask to preserve group write permissions.

refs #4444
LivestatusListener: Set unix socket permissions to 0660.

refs #4444
Install /var/run/icinga2/rw command mode, add configure options.

- changed locations for icinga2.cmd and livestatus unix socket
- make install creates /var/run/icinga2/rw and sets g+s for
  icinga:icingacmd
- configure options: --with-icinga[cmd]-{user,group}=
- add autoconf macros to check user/group and bail early if not created
- update documentation/INSTALL

fixes #4444
refs #3186