This is a nightmare situation to be in! Got it escalated in Microsoft who indeed know and have already fixed the source (so terminated backups don't trigger this) in the April CU http://blogs.technet.com/b/stefan_gossner/archive/2013/04/27/april-2013-cu-for-sharepoint-2013-has-been-released.aspx - you have to install March CU first.
However, it does not yet fix the fact that you can't switch the Site Collection back out of read-only, so is still a killer situation to be in, especially with a big site. Worse, it's no good restoring a new completed backup of the locked site coll (even though you know there's nothing actually wrong with it other than the read-only/maintenance-mode flag!) - as after all that, it remains locked! Fortunately though, the flag is at Site Coll level, not Farm, so even if it was your first backup (we had just migrated all the docs into it and go-live went pear-shaped thanks to this!) or you don't want to lose data with a recent backup, we found you can safely use Export-SPWeb to get everything out of that locked Site Coll, delete the Site Coll (painful but necessary!), create new and use Import-SPWeb to bring it all back again.
Tip - Import-SPWeb can just use the original filename of the Export and it will automatically chain together all those multiple .cmp files it split any large site into so you don't have to do them individually.
We've run it a day now and to our relief everything including the item-level permissions mapped to FBA accounts (you'll use -IncludeUserSecurity on the Export-SPWeb of course) is all running fine again. Heart attack over for now!
Microsoft called me back and gave me the solution!
PS C:\Users\root> $Admin = new-object Microsoft.SharePoint.Administration.SPS
PS C:\Users\root> $Admin.ClearMaintenanceMode()
PS C:\Users\root> $site.MaintenanceMode
After that, my site collection is not read-only any more!