make: add *.h as deps for *.o
undup-core: factor out stub_refresh
util: die(): write to f_debug as well.
add large content test
t.002.truncate: test parallel looped truncate
core, debug: fix -Wall warnings

 - remove unused variables
 - u8 rather than char as appropriate
 - prototype debug funcs
util.c: add threadids to verbose() and debug() and fix verbose() output stream
core: log hdr.len in stub_open
util: fix threadid to be unsigned
util: include pthread.h for pthread_self prototype