crypto/dsa: don't truncate input hashes.

crypto/dsa: don't truncate input hashes.

Although FIPS 186-3 says that we should truncate the hashes, at least
one other library (libgcrypt) doesn't. This means that it's impossible
to interoperate with code using gcrypt if we enforce the truncation
inside of crypto/dsa.

This change shouldn't actually affect anything because nearly
everybody pairs DSA with SHA1, which doesn't need to be truncated in
either case.

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/5471043