added new option "ServerX.Retention" to define server retention time (days); files older than configured server retention time are not even tried on this server

git-svn-id: svn://svn.code.sf.net/p/nzbget/code/trunk@1179 b1b436ca-2b18-4f33-ab5d-510789358d54
options "UnrarCmd" and "SevenZipCmd" can include extra switches to pass to unrar/7-zip. This allows for easy passing of additional parameters without creating of proxy shell scripts.

git-svn-id: svn://svn.code.sf.net/p/nzbget/code/trunk@1180 b1b436ca-2b18-4f33-ab5d-510789358d54