kvs: watch cb() should return integer

A watch callback should be able to terminate the reactor by returning
-1 like the other reactor callbacks.
kvs: update kvs_watch callback type in libzio
kvs: fix kvs_watch callback type in modctl
kvs: fix kvs_watch callback type in live
kvs: change kvs_watch callback type in lua binding

The internal watch callback always returns succcess.
Need to update Lua flux api to reflect this change.
kvs: update kvs_watch callback type in tkvswatch
Merge pull request #77 from garlick/kvswatch

kvs: modify kvs_watch callback type to return integer