go.crypto/blowfish: allow shorter passwords.

bcrypt didn't allow one, two and three letter passwords which is a
policy decision best left to the user of the code. Some users have
legacy issues which require such short passwords to be processed.

LGTM=bradfitz
R=golang-codereviews, bradfitz
CC=golang-codereviews
https://golang.org/cl/81800044
3 files changed