round-robin recording of chunks with seemless transition.

 * util.c: New set of clock functions to access the real time clock;
 * util.c: Split OpenFile and OpenAuxFile into several helper functions;
 * util.c: New function LookupAuxFile to find a position in an aux file;
 * util.c: New set of functions for directory input/output;
 * util.h: Fix a bug in the RTP header size calculation;
 * offsets.c: Moved functionality to multicat (util.c); offsets is still
   supported with the same scope, but is deprecated;
 * ingests.c: Reflect util.c API changes;
 * multicat.c: Modular input/output init functions;
 * multicat.c: Use poll() so that signals and duration option (-d) be taken
   into account even in the absence of an input stream;
 * multicat.c: New functions for directory output;
 * multicat.c: New options -k (initial seek into file or dir, possibly
   negative for 'NOW - value'), -r (round-robin chunk size of directory
   input/output) and -f (output packets as fast as possible without
   sleeping).

Typical usage for record:
$ mkdir test
$ multicat @239.255.255.255:5004 test &

For playout with one-minute timeshift:
$ multicat -k -$((27000000*60)) test 239.255.255.254:5004

Please note that expiration is not performed by multicat.