crypto/rsa: refactor RSA-PSS signing and verification

Cleaned up for readability and consistency.

There is one tiny behavioral change: when PSSSaltLengthEqualsHash is
used and both hash and opts.Hash were set, hash.Size() was used for the
salt length instead of opts.Hash.Size(). That's clearly wrong because
opts.Hash is documented to override hash.

Change-Id: I3e25dad933961eac827c6d2e3bbfe45fc5a6fb0e
Reviewed-on: https://go-review.googlesource.com/c/go/+/226937


Run-TryBot: Filippo Valsorda <filippo@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Katie Hockman <katie@golang.org>