bus: switch kdbus bloom filter over to SipHash (from MurmurHash3)

Let's try to standardize on a single non-cryptographic hash algorithm,
and for that SipHash appears to be the best answer.

With this change there are two other hash functions left in systemd: an
older version of MurmurHash embedded into libudev for the bloom filters
in udev messages (which is hard to update, given that the we probably
should stay compatible with older versions of the library). And lookup3
in the journal files (which we could replace for new files, but which is
probably not worth the work).

README

@@ -31,7 +31,8 @@ AUTHOR:
 LICENSE:
         LGPLv2.1+ for all code
         - except sd-daemon.[ch] and sd-readahead.[ch] which are MIT
-        - except src/shared/MurmurHash[23].c which is Public Domain
+        - except src/shared/MurmurHash2.c which is Public Domain
+        - except src/shared/siphash24.c which is CC0 Public Domain
         - except src/journal/lookup3.c which is Public Domain
         - except src/udev/* which is (currently still) GPLv2, GPLv2+