Centralise 'no random numbers' stubs into norand.c.

This is a small cleanup that removes a couple of copies of some boring
stubs, in favour of having just one copy that you can link against.

Unix Pageant can't currently use this, because it's in a precarious
state of _nearly_ having a random number generator: it links against
sshprng but not sshrand, and only uses it for the randomised keypress
acknowledgments in the GUI askpass prompt. But that means it does use
uxnoise, unlike the truly randomness-free tools.