Use misc methods of avoiding unnecesary header includes.
Replace int typedefs with int##_t from stdint.h.
Replace PRI64[xdu] with PRI[xdu]64 from inttypes.h.
Normalize QT_VERSION ifs where possible.
Resolve some indirect dependencies as direct ones.
Remove extern declarations from .cpp files.
This adds an executable `bitcoin-rpc` that only serves as a Bitcoin RPC
client.
The commit does not remove RPC functionality from the `bitcoind` yet,
this functionality should be deprecated but is left for a later version
to give users some time to switch.