fix copying of oneshot sha1 context. also decrease the size
of "small buffer", to make context smaller (and context copy
faster in most cases); it should be still enough to catch
most hmac operations.
there was a bug sha context copying, that caused all hmac users
(and possibly others) to crash. also implemented a third
intermediate hashing mode with small buffer: it'll speed up
hashing in most cases considerably (packets / certificates to
be hashed are not usually too long).
