The C library treats the recovery ID as trusted input and crashes
the process for invalid values, so it needs to be verified before
calling into C. This will inhibit the crash in #1983.
Also remove VerifySignature because we don't use it.
Linux build documentation is mostly geared towards Ubuntu 14.04 (LTS).
Appropriate package is called `libgmp-dev` there.
Note that on pristine installations building `geth` with godep will
fail because this header is missing. This is not documented in the
top-level README, but is on the wiki:
https://github.com/ethereum/go-ethereum/wiki/Installation-Instructions-for-Ubuntu#building-geth-command-line-client
That page recommends `libgmp3-dev`, which ATM provides same version
as `libgmp-dev`.
* Move random entropy functions to new package randentropy
* Add function to get n bytes entropy where up to first 32
bytes are mixed with OS entropy sources
