We were using intermediate values int he enum and casting back and forth
with a u32; it is cleaner to just use a u32 and define some special
values.
Silences ICC warning #188: enumerated type mixed with another type.
Bring hs_reset_stream(), hs_reset_and_copy_stream()'s functionality into
line with hs_close_stream() by accepting a NULL scratch if and only if
the match callback is also NULL, indicating that no matches should be
delivered.
