RPC client: Simplify command line string-to-JSON-value conversion code

By default, all command line parameters are converted into JSON string
values.  There is no need to manually specify the incoming type.

A binary decision "parse as string or JSON?" is all that's necessary.

Convert to a simple class, initialized at runtime startup, which offers
a quick lookup to answer "parse as JSON?" conversion question.

Future parameter conversions need only to indicate the method name
and zero-based index of the parameter needing JSON parsing.
Fix the build for Qt5

Merging #3883 broke the Qt5 build, define the color in the standard
way.
Merge pull request #4415

e35b37b RPC client: Simplify command line string-to-JSON-value conversion code (Jeff Garzik)
init.cpp: log fee estimates filename on error

- small changes to Shutdown(), buffer __func__, which is now used in
  all LogPrintf() calls and format for better readability
- order using namespace alpabetically