bitcoin-cli, rpcclient: prefer EXIT_FAILURE cstdlib constant

A more complex construction via abs() yields the same end result.
Add new univalue JSON handling module to libbitcoin_common.a
Add new "rawtx" command line utility

This is a simple utility that provides command line manipulation of
a hex-encoded TX. The utility takes a hex string on the command line
as input, performs zero or more mutations, and outputs a hex string
to standard output.

This utility is also an intentional exercise of the "bitcoin library"
concept. It is designed to require minimal libraries, and works
entirely without need for any RPC or P2P communication.