Issue #1865 - Clean up RPC help messages

Based on the proposal, update the help message of rpc methods
- strings arguments are in double quotes rather than square brackets
- numeric arguments have no quotes (and no default value)
- optional parameters are surrounded by round brackets
- json arguments are strings but don't use double quotes

Added 3 sections for the details
- Arguments: lists each argument, it's type, required or not, a default, and a description
- Result: The method result, with json format if applicable, type, and a description
- Examples: examples calls using bitcoin-cli and curl for json rpc call

Problems
- maybe this is too verbose
- lines might be too long
- description are not good or complete
- examples may be too much
Use C99 printf statements in mingw

Otherwise, format specifiers such as %llu will not work on XP or
earlier.
This bug was introduced with 51ed9ec9.
http://sourceforge.net/apps/trac/mingw-w64/wiki/gnu%20printf
tests: add testcases for strprintf
configure: lupdate is not needed for Qt build

Don't require lupdate to build with Qt. Like xgettext it is only
needed to update translations, not for normal builds.

This fixes the gitian build (broken by fcfbf547d).