Make sure that GetRandomBytes never fails

We're using GetRandomBytes in several contexts where it's either
unwieldy to return an error, or an error would mean a fatal exception
anyhow.

@gmaxwell checked OpenSSL a while ago and discovered that it never
actually fails, but it can't hurt to be a bit paranoid here.
gather_inputs: use correct variable in error message

"amount" and "fee" do not exist (anymore?).
Do not use third party services for IP detection.

This is a simplified re-do of closed pull #3088.

This patch eliminates the privacy and reliability problematic use
of centralized web services for discovering the node's addresses
for advertisement.

The Bitcoin protocol already allows your peers to tell you what
IP they think you have, but this data isn't trustworthy since
they could lie. So the challenge is using it without creating a
DOS vector.

To accomplish this we adopt an approach similar to the one used
by P2Pool: If we're announcing and don't have a better address
discovered (e.g. via UPNP) or configured we just announce to
each peer the address that peer told us. Since peers could
already replace, forge, or drop our address messages this cannot
create a new vulnerability... but if even one of our peers is
giving us a good address we'll eventually make a useful
advertisement.

We also may randomly use the peer-provided address for the
daily rebroadcast even if we otherwise have a seemingly routable
address, just in case we've been misconfigured (e.g. by UPNP).

To avoid privacy problems, we only do these things if discovery
is enabled.
Adding license.
Adding const.
Merge pull request #5249

38c62ae Adding const. (sinetek)
Do signature-s negation inside the tests

To avoid the need for libsecp256k1 to expose such functionality.
Merge pull request #5239

65e3a1e Make sure that GetRandomBytes never fails (Wladimir J. van der Laan)
Merge pull request #5246

a8a5e01 Adding license. (sinetek)
Don't show wallet options in the preferences menu when running with -disablewallet
Merge pull request #5245

d52f072 Don't show wallet options in the preferences menu when running with -disablewallet (sinetek)
Merge pull request #5240

5e2dcae gather_inputs: use correct variable in error message (dexX7)
Merge pull request #5161

845c86d Do not use third party services for IP detection. (Gregory Maxwell)
Merge pull request #5256

f4e0aef Do signature-s negation inside the tests (Pieter Wuille)