[htsp] autorec support and other improvements

Now it's possible for htsp clients to get/set autorecs and retrieve some other usefull DVR data.
Clients like ios, android apps and xbmc will benefit from this.

It's also possible to set the dvr retention time (like any other pvr software can do).
Pre and Post times are added to autorecs so clients can set a global pre and post times for all dvr entries in a autorec. This is also the case for the dvr retention time.

@perexg
Can you take a look?

I may made some mistakes with the inode stuff, it was much easier before the rewrite :p
Remove this whole block. If you don't define the "set" callbacks, the time_t or int operations are used by default - see src/prop.c . It's just a duplicate code.