In addition to fitting better (brute library is the verb, creds library
is the noun), this will allow creds.lua to use creds.Account internally
where necessary (see subsequent commits)
Also change old references to string argument "OPEN" into
creds.State.VALID.
Changes fall into these categories:
1. Avoid pathological string building. Loops over x = x .. "foo" can
become very slow. Instead, use strbuf.lua, table.concat, or just one
continuous concatenation; a = x .. y .. z is one operation, better than
a = x .. y; a = a .. z
2. Use hex-escaped strings instead of string.char. I find this more
readable in many cases, and it avoids a table lookup and function call.
3. Don't duplicate code. A few libraries and scripts had re-implemented
stdnse.generate_random_string or openssl.rand_bytes.
