rsyslog.logrotate: allow shell fragment in postrotate

Hi! the way log rotation is performed in this package makes it very hard for packages like orphan-sysvinit-scripts and runit-services to support rsyslogd. (this is also reported in #1031399); the problem is that currently log files are rotated by logrotate but rsyslogd does not receive HUP so it continues to write to rotated filesinstead of write to new ones.

The proposed fix turn /usr/lib/rsyslog/ in a directory where other packages can drop snippets. This doesn't change anything for a systemd system but allow packages like orphan-sysvinit-scripts and runit-services to drop their snippets to HUP rsyslogd