Some cleanups from NetBSD:

• C99 initializers.
• Change the default volume label from "NO NAME" to "NO_NAME".
• Set OEM String to "BSD4.4 " following the unnamed spacing convention in that other OS that suggests "MSWIN4.1"

Also, David Naylor's changes for Clang, mostly changing the signess
of constants.

Submitted by: Pedro F. Giffuni <giffunip tutopia com>
Clang fixes by: David Naylor <naylor.b.david gmail com>
Reviewed by: bde (with some disagreement about Clang issues)
MFC after: 2 weeks