rawtx: improve various user guards / command limits based on github fb
rawtx: Add ability to load JSON data into arbitary registers

This will enable more complex commands.

Also:
- add scattered comments
- improve CTxTn() construction
Makefile.am: move coins and keystore modules to libbitcoin_common.a

rawtx will use them for transaction signing.  They are sufficiently
low level to permit easy and direct movement between build ar archives.
rawtx: introduce transaction signing command

Update univalue as needed to provide query services.