Remove 'unsalted' PSS handling

[lumag]

The 'unsalted' PSS handling is not behaving like one would expect. It doesn't use salt of 0 length, but insteaad it uses some defaults which are not properly documented. Make the salt_len mandatatory for both singing and verification.

[lumag]

@tarcieri sounds good to me.

cc @roblabla @dignifiedquire

[tarcieri]

@lumag alternatively, you could implement ^^^ proposed API in this PR, which would avoid the need to deprecate anything.

Instead, change new to be salted, but infer the salt size from the digest function.

(Or I'm still happy to do that as a followup, which would simplify the overall changes and help organize discussion perhaps)

[lumag]

Sure, I'll do that in a minute

[lumag]

done